There are distinctive modern implementations of the abacus. Some designs, like the bead frame consisting of beads divided into tens, are used mainly to teach arithmetic, although they remain popular in the post-Soviet states as a tool. Other designs, such as the Japanese soroban, have been used for practical calculations even involving numbers of several digits. For any particular abacus design, there are usually numerous different methods to perform calculations, which may include the four basic operations, and also square and cube roots. Some of these methods work with non-natural numbers (numbers such as and ).