The Aztecs, or Mexica as they called themselves, were the elite of a militaristic empire centered at Central Mexico when the Spanish conquistadores landed in America at the beginning of the 16th century CE. The Aztecs originated in the semi-arid environments of northern Mexico as one of the many barbarian or "Chichimec" tribes. When they arrived at the fertile Valley of Mexico at 14th century CE, they found the land already settled and divided by city states. They built their city, Tenochtitlan, in the marshes of Lake Texcoco, and quickly adopted much of the culture of their new neighbors. As a result, the Aztecs adopted a writing system that had been used for many centuries before and shared by many of the other nations of Central Mexico.
The language that the Aztec spoke was called Nahuatl. It was also the language of the majority of the people in Central Mexico and a lingua franca in large parts of Mesoamerica. The origin of Nahuatl writing is poorly understood. It most resembles Mixtec writing in that both use dots for numbers less than twenty (in contrast to the bar-and-dot notation used in Maya and Zapotec), share similar construction style of compound signs, and place emphasis on short texts that rely on painted scenes for narratives instead of longer texts. It is thought that Mixtec writing influenced Nahuatl writing, but both are possibly influenced by earlier writing systems of cities such as Xochicalco, Cacaxtla, and the even more ancient Teotihuacan.
Nahuatl writing was primary written on perishable media such as deer-skin and paper codices. Due to ravages of time and purposeful destruction of books by both the Aztecs and the Spanish conquistadores, no pre-Columbian book has survived to the modern age. All surviving documents containing Nahuatl writing were composed after the Conquest and contained a mixture of Aztec glyphs and Spanish notes. There are a few codices made before the Conquest from the Puebla region in a somewhat different style known as the "international" Mixteca-Puebla, style, but their exact relationship to either Aztec or Mixtec writing is still somewhat obscure.General Overview
Nahuatl writing had three primary functions, namely to mark calendrical dates, to record accounting mathematical calculations, and to write names of people and places. No continuous texts like those of the Maya, Epi-Olmec, or even Zapotec writing system has been be found. As noted earlier, pictorial representation of events are used in lieu of long texts to record history.
Like other Mesoamerican scripts, the core of Nahuatl writing consists of a set of calendrical signs and a vigesimal number system. The most important calendrical cycle observed by the Aztecs was the 260-day sacred calendar, called tonalpohualli in Nahuatl. The tonalpohualli is essentially two parallel and interlocking cycles, one of 20 days (represented by "day signs"), and one of 13 days (represented by numbers called "coefficients"). The following are the 20 day signs in the Aztec sacred calendar. The Nahuatl names are in red, and their meanings in English are in blue.
A date in the tonalpohualli is composed of a day sign and and a coefficient. So, for example, the first day in the 260-day cycle would be 1 Cipactli. As both the day sign and the coefficient moves forward, the next day would be 2 Ehecatl. This goes on until 13 Acatl is reached, at which point the coefficient cycle loops back to 1, and hence the next day would be 1 Ocelotl. Similarly, upon reaching the last day sign on day 7 Xochitl, the day sign cycle goes back to the first sign, and the next day would be 8 Cipactl.
The Aztecs had a 365-day solar calendar called xiuhpohualli, which consisted of 18 months of 20 days, and an unlucky 5-day period at the end of the year. However, they rarely recorded dates in the solar calendar on manuscripts, and never on monuments.
In addition, like other Mesoamerican cultures, the Aztecs also employed the Calendar Round, a 52-year period created by interlocking the 260-day and 365-day cycles. A year in the Calendar Round was named by the tonalpohualli name of last day of the last month in the xiuhpohualli for that year. Because of the way the math worked out, only four day signs, namely Calli, Tochtli, Acatl, and Tecpatl, could be part of a year's name, and hence they were called "year bearers". Accompanying the year bearers were coefficients, which could range from 1 to 13. To distinguish Calendar Round years from days in the 260-day calendar, years glyphs were drawn inside rectangular "cartouches". A good example occurs in the Codex Telleriano-Remensis, a document written after the Spanish Conquest but at a time when knowledge of the pre-Columbian culture was still available. In this document, Aztec years are correlated to Western Gregorian years.
As you have probably gathered by now, Aztec numbers are represented by long sequences of dots. In general, the Aztecs almost exclusively used dots on manuscripts as well as on stone monuments, but the more ancient bar-and-dot system does make rare appearances on carved monuments as well, primarily due to artistic consideration. The dot system, while feasible for calendrical use (since no number will ever exceed 20), was impossible when dealing with accounting, especially since the Aztec Empire had to record large amounts of tribute frequently demanded from its provinces. The Codex Mendoza, another post-Conquest manuscript, depicted life in Central Mexico around the time of conquest and also contained a section on the tribute exacted by the Empire. To count items in excess of 20 efficiently, the Aztecs used glyphs for the numbers 20 (a flag), 400 (a feather), and 8000 (a bag of incense).
For example, the number 500 would be a feather and five flags (400 + 5 x 20 = 500). To indicate that the multiple glyphs forming a number belong to a single sign group, a line is drawn to connect all the glyphs. The line is then connected to the object it is counting.
The previous examples are taken from the Codex Mendoza, and they provide both the Aztec and Spanish versions of the information they are conveying. On the left, you can see the a bundle topped by a series of five flags, which is the number 100 (5 x 20), and is reflected by the Spanish caption "çient cargas de cacao", meaning "100 loads of cacao beans". In the middle, the Aztec representation is that of four flags and a bird, which is mirroed in the Spanish caption "ochenta pieles de pajaros deste color", or "80 pelts of birds of this color". And finally, on the right, the caption "cccc manojo de plumas coloradas" meant "400 bundles of red feathers" and is shown in Aztec as a schematic, black feather (400) with a bundle of red feathers.
In addition to calendrical and numeric signs, a number of highly pictorial logograms were used to write down personal names, names of places, and historical events. For example, there are many records of the Aztec army conquering other cities documented in the Codex Mendoza. To show that a city has been conquered, the city's name is written next to the "conquered" glyph which is a temple (pyramid) in smoke and flames with its top toppling over. In the following example, the ancient cities Colhuacan and Tenayucan were shown to be conquered. And to drive the point home, Aztec warriors are shown with captives taken from these conquered cities.
The Nahuatl language is polysynthetic, which means that compound words and long phrases are constructed from roots and affixes. Reflecting this characteristic, Aztec names are often written as groups of highly pictorial logograms that make up the roots of the name. The glyphs are joined together or even sometimes combined into a single glyph (a process called conflation) to show that they form a compound word. The following are a number of examples of logograms forming glyph blocks. Note that logograms are transcribed using bold upper case letters in their root forms, meaning the unchanging part of the word. Also note that Nahuatl nouns are transcribed in italicized lower case letters in the absolutive case, which is the root plus an ending of either -tl, -tli, or -li. While too longer to explain here, the absolutive case in Nahuatl can be thought as the singular subject of a sentence.
While a lot of Nahuatl names can be represented by joining logograms together for their meaning, sometimes it is difficult to visually depict a concept graphically. To solve this, in certain cases a logogram is used for its phonetic value rather than its meaning in order to represent another root, suffix, or syllable(s) that sounds identical or similar to the logogram. This process is called rebus writing and is also quite productive in Aztec writing, as you can see in the following examples.
Rebus writing is one of the ways to represent sounds rather than meanings in Nahuatl writing. In addition, a number of Nahuatl logograms can also function as single-syllable phonetic signs. And in fact, like rebus writing, their phonetic values are derived from the words the logograms represent. However, unlike rebus writing which uses the full sound of the root, phonetic glyphs are always single-syllable and take their value from the first syllable (minus any ending consonants) of the word the logograms represent. This is called the acrophonic principle and is found throughout writing systems of the world.
The following is a chart of Nahuatl glyphs with known phonetic values.
For example, the glyph a is derived from a-tl "water", tla is a set of teeth and based on the word tlan-tli "tooth", and ko which is from comi-tl "pot". Note that while logograms are transcribed using upper-case letters, phonetic signs are transcribed with lower-case letters.
Often names found in Aztec manuscripts and monuments included a good amount of phonetic glyphs. Sometimes the phonetic glyphs serve to complement logograms, in that they spell out one or more syllables of the logogram. Sometimes the glyphs spell out whole syllables without the presence of logograms. And somewhat more rarely, the entire name is written out fully phonetically.
One might find that from the previous examples the Aztec writing system is complicated and not straightforward to modern eyes. Glyphs can be polyvalent, functioning as both logograms and phonetic signs, and are not always read in a linear fashion but could jump from one end to another. However, Aztecs and their neighbors produced countless numbers of manuscripts with subject matters as diverse as time-keeping, astrology, mythology, genealogy, and history. While these manuscripts were highly pictorial, intimate knowledge of the underlying language, Nahuatl, was absolutely essential to fully interpret the glyphs. In this regard, the Aztec glyphs truly constituted an active and productive writing system.