CAP, in ship-building, a strong thick block of wood, used to confine two masts together, when one is erected at the head of the other in order to lengthen it. It is for this purpose furnished with two holes perpendicular to its length and breadth, and parallel to its thickness: one of these is square, and the other round: the former being solidly fixed upon the upper end of the lower mast, whilst the latter receives the mast employed to lengthen it, and secures it in this position.