Generally, Persian rugs are classified by the region in which they are made. For example, a rug would be identified in the market as a Tabriz if it was woven in or near the city of the same name. A major rug-producing center such as Tabriz may also have subcategories, such as the famous Tabriz Mahi.
Every city, village, or tribe has their own design that they incorporate into their rugs, much like a trademark. If a design becomes famous, other centers may attempt to imitate the design though it seems imitators never quite match the quality of the original.
Persian rugs aren't always from the city they are identified as quality is also an identifying factor. For example, in the holy city of Mashad, when finer pieces are woven, they are often referred to as Moods. Although Mood is a village near Mashad, the name doesn't necessarily mean that the rug was woven there.
Another way Persian rugs are classified is by the city in where they are marketed. For example, Arak is a small rural town that has an incredibly large rug trading industry. It is surrounded by dozens of other villages that all produce rugs of their own. All these rugs are marketed in Arak, and thus many of them are referred to as Araks.
Likewise Birjand is a town in the province of Khorassan a fair distance south of Mashad, but some rugs of a certain quality marketed through Mashad are referred to as Birjands. This system of geography and quality of the Persian rug industry is common throughout Iran.
Typically there are two types of rugs made in Iran: tribal rugs and city rugs. Tribal rugs are those woven by nomads and inhabitants of small rural villages. For the most part, these rugs are inferior in quality to the ones made in the cities. However the materials such as the wool and dyes used are often of excellent quality and occasionally a tribal rug turns out surprisingly fine.
The dyes used in tribal rugs are still mainly natural vegetable dyes, which is superior to chemical or chrome dyes.
There are many different designs found in Persian rugs. Tribal rugs tend to have geometric designs with little detail, only a few bright colors. City rugs and finer pieces usually have a more detailed design and much more color. Also, some cities will have very few designs and others such as Tabriz will have a great number. A rug expert can usually determine the origin of a rug simply by analyzing the design.
Persian rugs are categorized by quality first and design second. Quality refers mainly to the knotting of the rug plain and simple: the higher the knot count, the higher the quality. Other factors that contribute to the grading of rugs are the quality of the wool or silk, the dyes used, and the symmetry and accuracy throughout the design.
You may come across coarse Persian rugs that aren't perfectly square or contain a main color that varies in tone from one end of the rug the other. These imperfections, however, are what give these rugs their character and authenticity. A machine made rugs may be perfectly square, but the quality of these mass-produced rugs is otherwise inferior to handmade rugs in every aspect.
Surprisingly, a fine Persian rug will almost always include intentional imperfections. In fact, there's an old Persian proverb that says, "A Persian Rug is Perfectly Imperfect, and Precisely Imprecise". This notion of intentionally including slight and minor irregularities is derived from the religious belief that God is the only perfect being and that attempting absolute perfection would be claiming the position of the Almighty.
The most popular sizes for Persian rugs are seven by ten feet, eight by twelve feet, and ten by thirteen feet. These standard sizes each have names in Iran, and most regions will usually produce only one size. In larger centers, however, you can find rugs of non-standard dimensions ranging from a small two by three foot mat to carpets as large as a city block.
You can also find runners for hallways and corridors ranging from five to thirty feet long, and some cities even produce round or oval carpets.
The Iran Carpet Company, a specialist in the subject, has attempted to classify Persian carpet designs and has carried out studies of thousands of rugs. Their results show that there have been slight alterations and improvements to almost all original designs.
In its classification the company has called the original designs as the 'main pattern' and the derivatives as the 'sub patterns'. They have identified 19 groups, including: historic monuments and Islamic buildings, Shah Abbassi patterns, spiral patterns, all-over patterns, derivative patterns, interconnected patterns, paisley patterns, tree patterns, Turkoman patterns, hunting ground patterns, panel patterns, European flower patterns, vase patterns, intertwined fish patterns, Mehrab patterns, striped patterns, geometric patterns, tribal patterns, and composites.