A mast cap is a strong, thick block of wood, used to confine two masts together, when the 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 it's length and breadth, and parallel to it's 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.
The principal caps of a ship are those of the lower-masts, which are fitted with a strong eye-bolt on each side, wherein to hook the block by which the top-mast is drawn up through the cap.
The breadth of all caps is equal to twice the diameter of the top-mast, and the length to twice the breadth. The thickness of the main and forecaps is half the diameter of their breadths; the mizen-cap three-sevenths, and the top-mast-caps two-fifths of their respective breadths.
In the same manner as the top-mast slides up through the cap of the lower-mast, the top-gallant-mast slides up through the cap of the top-mast.