Around Rome, I noticed many numbered tiles (most with four digits, and all with a white background and blue text) around the city. An example is shown below on the right of the image.
Some of these tiles were placed at the entrances of historically important buildings (for example, Santi Quatro Coronati has a numbered tile), but many were at addresses with no discernable importance. I also verified that the number on the tile did not correspond to the street number. (This is evident, for example, for the property in the picture above; the street number is 28).
What is the significance of locations with these tiles? Is there a system behind their numbering / a list of all the locations of the tiles? More images are available in this album.
What is the significance of numbered tiles on walls in Rome? - History
Centered in the city of Rome, the civilization of Ancient Rome ruled much of Europe for over 1000 years. The arts flourished during this time and were often used by the wealthy and powerful to memorialize their deeds and heritage.
The Romans admired the Greek culture and arts. After conquering Greece, they brought many Greek artists to Rome to make sculptures for them in the Greek fashion. The art of Ancient Greece had a great influence on the art of Ancient Rome.
Although Greek art had the greatest influence on the Romans, other civilizations that they conquered and encountered over their wide empire also had influence. These included the Ancient Egyptians, eastern art, the Germans, and the Celtics.
Roman sculpture played an important part of the Roman daily life. Sculptures took the form of full statues, busts (sculptures of just a person's head), reliefs (sculptures that were part of a wall), and sarcophagi (sculptures on tombs). The Ancient Romans decorated with sculptures in a number of places including public buildings, public parks, and private homes and gardens.
Roman sculpture was heavily influenced by Greek sculpture. In fact, many of the Roman sculptures were just copies of Greek sculptures. The wealthy Romans decorated their large homes with sculptures. A lot of times these sculptures were of themselves or their ancestors. Other popular subjects for sculptures included gods and goddesses, philosophers, famous athletes, and successful generals.
Above is a marble statue of Augustus the first Emperor of Rome. He is shown here wearing a traditional Roman toga while performing his duties as Pontifex Maximus.
One of the most popular types of sculpture in Ancient Rome was the bust. This is a sculpture of just the head. Wealthy Romans would put the busts of their ancestors in the atrium of their homes. This was a way for them to show off their lineage.
Bust of Vibia Sabina by Andreas Praefcke
The walls of the homes of wealthy Romans were often decorated with paintings. These paintings were frescos painted directly on the walls. Most of these paintings have been destroyed over time, but some of them were preserved in the city of Pompeii when it was buried by the eruption of a volcano.
Painting discovered on a wall in the ruins of Pompeii
Source: The Yorck Project
The Romans also made pictures from colored tiles call mosaics. The mosaics have been able to survive the test of time better than the paintings. Sometimes the tiles would be applied direct at the site of the mosaic. Other times the tiles and the base would be made in a workshop and the entire mosaic installed later. Mosaics could be art on a wall, but also worked as decorative flooring.
After the Middle Ages, the artists of the Renaissance studied the sculptures, architecture, and art of Ancient Rome and Greece to inspire them. The classic art of the Romans had a significant influence on art for many years.
Traditional mosaics are made of cut small cubes of roughly square pieces of stone or hand-made glass enamel of different colors, known as tesserae. Some of the earliest mosaics were made of natural pebbles, originally used to reinforce floors. 
Mosaic skinning (covering objects with mosaic glass) is done with thin enameled glass and opaque stained glass. Modern mosaic art is made from any material in any size ranging from carved stone, bottle caps, and found objects.
The earliest known examples of mosaics made of different materials were found at a temple building in Abra, Mesopotamia, and are dated to the second half of the 3rd millennium BC. They consist of pieces of colored stones, shells, and ivory. Excavations at Susa and Chogha Zanbil show evidence of the first glazed tiles, dating from around 1500 BC.  However, mosaic patterns were not used until the times of Sassanid Empire and Roman influence.
Greek and Roman Edit
Bronze Age pebble mosaics have been found at Tiryns  mosaics of the 4th century BC are found in the Macedonian palace-city of Aegae, and the 4th-century BC mosaic of The Beauty of Durrës discovered in Durrës, Albania in 1916, is an early figural example the Greek figural style was mostly formed in the 3rd century BC. Mythological subjects, or scenes of hunting or other pursuits of the wealthy, were popular as the centrepieces of a larger geometric design, with strongly emphasized borders.  Pliny the Elder mentions the artist Sosus of Pergamon by name, describing his mosaics of the food left on a floor after a feast and of a group of doves drinking from a bowl.  Both of these themes were widely copied. 
Greek figural mosaics could have been copied or adapted paintings, a far more prestigious artform, and the style was enthusiastically adopted by the Romans so that large floor mosaics enriched the floors of Hellenistic villas and Roman dwellings from Britain to Dura-Europos.
Most recorded names of Roman mosaic workers are Greek, suggesting they dominated high quality work across the empire no doubt most ordinary craftsmen were slaves. Splendid mosaic floors are found in Roman villas across North Africa, in places such as Carthage, and can still be seen in the extensive collection in Bardo Museum in Tunis, Tunisia.
There were two main techniques in Greco-Roman mosaic: opus vermiculatum used tiny tesserae, typically cubes of 4 millimeters or less, and was produced in workshops in relatively small panels which were transported to the site glued to some temporary support. The tiny tesserae allowed very fine detail, and an approach to the illusionism of painting. Often small panels called emblemata were inserted into walls or as the highlights of larger floor-mosaics in coarser work. The normal technique was opus tessellatum, using larger tesserae, which was laid on site.  There was a distinct native Italian style using black on a white background, which was no doubt cheaper than fully coloured work. 
In Rome, Nero and his architects used mosaics to cover some surfaces of walls and ceilings in the Domus Aurea, built 64 AD, and wall mosaics are also found at Pompeii and neighbouring sites. However it seems that it was not until the Christian era that figural wall mosaics became a major form of artistic expression. The Roman church of Santa Costanza, which served as a mausoleum for one or more of the Imperial family, has both religious mosaic and decorative secular ceiling mosaics on a round vault, which probably represent the style of contemporary palace decoration.
The mosaics of the Villa Romana del Casale near Piazza Armerina in Sicily are the largest collection of late Roman mosaics in situ in the world, and are protected as a UNESCO World Heritage Site. The large villa rustica, which was probably owned by Emperor Maximian, was built largely in the early 4th century. The mosaics were covered and protected for 700 years by a landslide that occurred in the 12th Century. The most important pieces are the Circus Scene, the 64m long Great Hunting Scene, the Little Hunt, the Labours of Hercules and the famous Bikini Girls, showing women undertaking a range of sporting activities in garments that resemble 20th Century bikinis. The peristyle, the imperial apartments and the thermae were also decorated with ornamental and mythological mosaics.  Other important examples of Roman mosaic art in Sicily were unearthed on the Piazza Vittoria in Palermo where two houses were discovered. The most important scenes there depicted are an Orpheus mosaic, Alexander the Great's Hunt and the Four Seasons.
In 1913 the Zliten mosaic, a Roman mosaic famous for its many scenes from gladiatorial contests, hunting and everyday life, was discovered in the Libyan town of Zliten. In 2000 archaeologists working in Leptis Magna, Libya, uncovered a 30 ft length of five colorful mosaics created during the 1st or 2nd century AD. The mosaics show a warrior in combat with a deer, four young men wrestling a wild bull to the ground, and a gladiator resting in a state of fatigue, staring at his slain opponent. The mosaics decorated the walls of a cold plunge pool in a bath house within a Roman villa. The gladiator mosaic is noted by scholars as one of the finest examples of mosaic art ever seen — a "masterpiece comparable in quality with the Alexander Mosaic in Pompeii."
A specific genre of Roman mosaic was called asaroton (Greek for "unswept floor"). It depicted in trompe-l'œil style the feast leftovers on the floors of wealthy houses. 
What is the significance of numbered tiles on walls in Rome? - History
The first of the four styles is called Incrustation and dates back from 200 to 60 B.C. This style is identified by colorful blocks painted on the wall to resemble large marble slabs. Often, the plaster on the wall is molded into raised backs, adding to the so-called marbleizing effect. The idea of faux-finishes is something that has traversed the ages and is still a popular decorating technique in todays houses. Although this style was often eye-catching and colorful in ancient Rome, it could often seem claustrophobic in the small rooms of Roman houses.
The second style, Architectural, dates from around 60 to 20 B.C. and serves to open up the limited space of Roman houses. This style has a distinctly realistic feel and tends to reflect everyday objects and scenery as they actually appear. Often times, illusions of windows and covered walkways lined with columns that looked onto imaginary scenes were framed by painted columns. Objects of daily life were depicted in such a way as to seem real, with metal and glass vases and shelves, and tables appearing to project out of the wall. A shift from public, realistic architecture to more private decoration is made as the third style develops.
The third style, Ornamental, dates from 20 B.C. to 20 A.D. In it, there is a closing up of space. Illusion is rejected in favor of ornamentation. Largely monochromatic walls were often painted with a few pieces of architecture. For instance, candelabra or slender columns were used to divide the wall into separate sections. These sections then supported smaller, framed paintings, set up in the fashion of an art gallery. One benefit of this style of wall paintings was that it gave artists and patrons more flexibility to change their designs. Instead of having to re-paint the entire wall, an artist could decide to just change one of the framed pictures if he so desired. As time progressed, the style of wall paintings became even less architecturally realistic and more of a mixture of styles.
The fourth style, Intricate, dates from around 20 A.D. to the fateful year of 79 A.D. when Mount Vesuvius erupted. This style incorporates all the elements of the earlier styles. It doesnt resemble any believable space, but instead consist of a variety of architectural elements arranged in an unrealistic manner with unrealistic perspective, set against a flat background. Unlike the clarity of the third style galleries, fourth style rooms appear chaotic and filled to excess.
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The Materials used in Colosseum:
- 1 – Travertine: The limestone which was used in the main pillars and walls were strong and brought from Tibur. It was generally white or yellow.
- 2 – Tuff: Tuff is softer than travertine and it brings elasticity. However since it was not resistant to the fire, the damage was bigger in cases of big fires in Colosseum.
- 3- Concrete: Concrete was invented by the Romans. It was cheap, quick to make, and easy to use. To make concrete, the Romans dropped pieces of rubble into sticky mortar made of lime (a powder of burned chalk or limestone), water, and pozzolana (a volcanic ash). The mortar hardened as it dried, holding the concrete together. Rubble gives concrete its strength. Big lumps of heavy rubble were used in the Colosseum’s foundations. Small lumps of light rubble were used in the upper walls.
- 4- Bricks: Bricks were mixed with water, sand and tiles.
- 5 – Iron / Bronze Clamps: In order to bind stones together these clamps were used.
- 6 – Marble: Marble is used both in decoration and the entrances of the cavea in Colosseum. Some of the columns are also made of marble. The first three marble rows were for the nobles and special guests.
- 7 – Lime: Lime was used as binder for the cement by adding water. It was made of limestone which is heated.
- 8 – Mortar: Mortar is mixture of cement/sand and water. There were two types of mortar: The first one is lime mortar – the one we mentioned above, and pozzolanic mortar – the volcanic ash which is an aluminous material reacted with calcium hydroxide.
- 9 – Stone: Stone was used on the outside walls of Colosseum and the sections of the building that took the most weight. Big lifting cranes hoisted stone blocks into the air. They were powered by roman slaves walking around and around a treadwheel. Inside walls of Colosseum and underground passages (called Hypogeum) were built using heavy concrete and brick. The upper levels were built with less heavy materials, such as wood, bricks, and the lightest concrete.
Interior of Colosseum, ancient Roman amphitheater. – Building Materials of the Colosseum –
Wind is one of two honor suits. These tiles each feature the character for compass directions: north (北, běi), east (東, dōng), south (南, nán), and west (西, xī). Like the characters simple suit, it is necessary to learn to read the cardinal direction characters in Chinese to recognize and organize this suit.
There are four sets, and each set has four tiles. The total number of wind tiles in each game set is 16.
The other honor suit is called arrows, or dragons. There are four sets of arrows tiles, and each set has three tiles. This threesome has several meanings which are derived from the ancient imperial exam, archery, and Confucius’ cardinal virtues.
One tile features a red 中 (zhōng, center). The Chinese character represents 紅中 (hóng zhōng), which connotes passing the imperial exam, a hit in archery, and the Confucian virtue of benevolence.
Another tile features a green 發 (fā, wealth). This character is a part of the saying, 發財 (fā cái). This saying translates to "get rich," but it also represents an archer releasing his or her draw and the Confucian virtue of sincerity.
The last character features a blue 白 (bái, white), which represents 白板 (bái ban, white board). The white board means freedom from corruption, a miss in archery, or the Confucian virtue of filial piety.
To finish the wall, each player selects an additional five tiles from the Well and places three tiles on the right side of the wall and two tiles on the left. Fiver more tiles are selected, and the second level is completed by adding three tiles on the right and two on the left.
At this point, there will be 17 tiles on the top and 17 tiles on the bottom with all the tiles in one row. There should be a total of four walls. If the mahjong game set includes tile racks, these can be used to straighten the walls and the separate walls may be joined together to make a rectangle.
12-Inch Square Tile
Zero Creatives/Getty Images
Tile measuring 12 inches by 12 inches represents the start of floor tile sizes. This is the smallest tile found in great abundance on floors. Smaller sizes can be found but in far fewer numbers. Twelve-inch square tile is a favorite of do-it-yourselfers as it is easy to handle and will fit in most snap tile cutters and wet tile saws.
Though used mainly on floors, this size of tile is sometimes found on walls.
Twelve-inch square tile is most often made from ceramic, porcelain, and stone.
Legend tells us that Remus had selected the Aventine to live on. It was there that he watched the bird omens, while his brother Romulus stood on the Palatine, each claiming the better results.
The Aventine is noteworthy for its concentration of temples to foreign deities. Until Claudius, it was beyond the pomerium. In "Foreign Cults in Republican Rome: Rethinking the Pomerial Rule", Eric M. Orlin writes:
The Aventine Hill became the home of the plebeians. It was separated from the Palatine by the Circus Maximus. On the Aventine were temples to Diana, Ceres, and Libera. The Armilustrium was there, too. It was used to purify arms used in battle at the end of the military season. Another significant place on the Aventine was Asinius Pollio's library.
This Preservation Brief was originally developed as a slide talk/methodology in 1982 to discuss the use of the Secretary of the Interior's Standards for Rehabilitation in relation to preserving historic character and it was amplified and modified in succeeding years to help guide preservation decision making, initially for maintenance personnel in the National Park Service.
A number of people contributed to the evolution of the ideas presented here. Special thanks go to Emogene Bevitt and Gary Hume, primarily for the many and frequent discussions relating to this approach in its evolutionary stages to Mark Fram, Ontario Heritage Foundation, Toronto, for suggesting several additions to the Checklist and more recently, to my coworkers, both in Washington and in our regional offices, especially Ward Jandl, Sara Blumenthal, Charles Fisher, Sharon Park, AIA, Jean Travers, Camille Martone, Susan Dynes, Michael Auer, Anne E. Grimmer, Kay Weeks, Betsy Chittenden, Patrick Andrus, Carol Shull, Hugh Miller, FAIA, Jerry Rogers, Paul Alley, David Look, AIA, Margaret Pepin-Donat, Bonnie Halda, Keith Everett, Thomas Keohan, the Preservation Services Division, MidAtlantic Region, and several reviewers in state preservation offices, especially Ann Haaker, Illinois and Stan Graves, AIA, Texas for providing very critical and constructive review of the manuscript.
This publication has been prepared pursuant to the National Historic Preservation Act of 1966, as amended, which directs the Secretary of the Interior to develop and make available information concerning historic properties. Technical Preservation Services (TPS), National Park Service prepares standards, guidelines, and other educational materials on responsible historic preservation treatments for a broad public.