Today’s rice comes directly or indirectly from three basic species, which are originally from India, Japan and from Java. Keeping in mind these characteristics it is easier to understand the general criteria which guide the classification of rice; these criteria follow standard, based on the work of the FAO (Food and Agricultural Organization) as set forth during three sessions of international experts, Bangkok (1955), Rome (1956) and Washington (1958).
In simple terms, the main characteristics of classification are as follows:

• Length
• Form
• Aspect

Length:

- measured in millimetres.
- by international convention, one distinguishes three categories:

• round (up to 5.2 mm)
• medium (up to 6 mm)
• long and oblong (relationship of length to width at least at 3:1)

- in Italy four classification is in use:

• round grain (comune or originario), up to 5.2 mm
• semifino (5.2 to 5.6 mm)
• superfino (over 6.2 mm)

Form:

Depending on the more or less elliptical form of the grain, one usually distinguishes between small or round, medium, large, very large. The form, by the way almost always readily discernible, is technically indicated by the weight of 1000 grains. Among Italian rice, the Arborio is of particular relevance, in as much as it is the only variety definable as very large (1000 grains weigh more than 35 g). Even internationally, varieties such as Arborio are rare. Under the category of large, other than Carnaroli, also the following varieties are well known, such as Roma, Baldo and S. Andrea. On the other hand, Vialone Nano belongs to the medium category, even if, once cooked, it comes near to the large category.