The invention relates to a seat (1), in particular an office chair, comprising a seat support (3) for a seating surface (6) and comprising a backrest support (4) of a backrest (5), which is held on the seat support (3) so as to be able to move in a tiltable manner to the side, wherein the backrest support (4) comprises two support aims (4a, 4b) which are configured to be flexible and/or articulated laterally and spaced apart from one another. Preferably, the seating surface (6) and/or the backrest (5) comprises a curved seat shell and/or backrest shell (6a, 5a) provided with a cover (6b, 5b) with a number of flexible curved ribs (15, 16) on the edge of the shell, wherein the cover is stretched by the seat shell and/or backrest shell (6a, 5a) such that when the cover (6b, 5b) is loaded, the bending of the curved ribs (15, 16) is altered.