Short timbers mounted vertically on each side of the ship for holding the main sheet lead blocks. The term also describes a sheave in the bulwark through which the main sheet roves.
• (n.) A piece of oak bolted perpendicularly on the side of a vessel, to aid in drawing down and securing the clew of the mainsail.
Chess'tree` noun [ Confer French chassis a framework of carpentry.] (Nautical) A piece of oak bolted perpendicularly on the side of a vessel, to aid in drawing down and securing the clew of the mainsail. 
(in the 17th and 18th centuries) a wooden fastening with one or more sheaves, attached to the topside of a sailing vessel, through which the windward tack of a course was rove.