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A bol is a mnemonic syllable. It is used in Indian music to define the tala, or rhythmic pattern, and is one of the most important parts of Indian rhythm. Bol is derived from the Hindi word bolna, which means "to speak."
Mnemonic syllables are used in a variety of classical drumming situations in India. They are found in the pakhawaj and the tabla. However since the pakhawaj has become rather rare, the term bol is mostly thought to apply to the tabla.
There is only a very loose connection between the bols and their technique. Sometimes different techniques are used because different musicians represent different gharanas, or musical styles. Sometimes different techniques are used in order to get a technical advantage (e.g., playing very fast). Sometimes different techniques are used simply because a different artistic statement is being made. For these reasons, it is impossible to categorically say that a certain bol will be executed in any one fashion.
In the tabla just as in the case of the mridingum and pakhawaj one who has learned the art of playing these drums is taught to recite what he is going to play. What is recited is said to be the bol. The recitation contains various consonant vowel sequences. It is expected that the actual rendering on the drum mimics the bol.