An additional part laced to the foot of the jibs, or other fore-and-aft sails, in small vessels in moderate weather, to gather more wind. They are commonly one-third of the depth of the sails they belong to. Thus we say, "Lace on the bonnet," or "Shake off the bonnet." Bonnets have lately been introduced to secure the foot of an upper-topsail to a lower-topsail yard. The unbonnetted sail is for storm service. Bonnet, in fortification, is a raised portion of the works at any salient angle, having the same plan, but 10 or 12 feet more command than the work on which it is based. It assists in protecting from enfilade, and affords a plunging fire.