Tar and Tonbak
The literal meaning of the term is 'four strokes' or 'four plucks'. This form is usually composed, but often contains improvised variations on repeated one- or two-bar melodic motifs.
Improvisation is less the case in modern written compositions, which also tend to be longer and more elaborate than the traditional shorter compositions. The metre may be triple (6/8) or duple (2/4 or 4/4); in this example, the metre is 4/4 changing to 6/8.
The chaharmezrab is not an ensemble piece, instead being in the nature of an etude intended to display the skill of the soloist. Here there is a moderate degree of improvisation.