Tar is a long-necked lute. Its sound-box is 8-shaped made of mulberry wood, covered with a skin parchment functioning as its sound board.
Tar had three double courses of metal strings tuned in C-G-C. The neck has 25-28 movable frets made of catgut or nylon. Since bridge is positioned on the parchment, the instrument is so sensitive to temperature changes and easily goes out of tune.
A brass pick inserted in a wax and ash ball is used for plucking.
Tar is the most important classical instrument and the most suitable for explaining the theoretical foundations of Persian classical music-its system of intervals and modes.
In this way it is very similar to piano in western classical music. The most important version versions of radif, i.e. Mirza Abdollah and Aqa Hoseyn-Qoli's versions, both were originally conceived on tar, the principal instrument of the Farahani family. Its playing technique has remarkably improved and nowadays there are numerous tar virtuosos.
Source: An Introduction to Iraninan music booklet from Mahoor Institute of Culture and Art