Four-sided sails: gaff, gunter, lug and sprit

Before the bermudan triangle took over, mainsails had four sides: a spar held the top edge up, so the sail carried area without a tall mast. The forward top corner is the throat, the after one the peak.

Gaff — the classic: a gaff spar rises aft at a moderate angle, putting lots of area low down. Gunter — the yard stands almost vertical, effectively extending a short mast, so the whole rig fits inside the boat for trailing. Lug — the yard is steeper and set further forward, crossing the mast. Sprit — a diagonal spar props the peak up from low on the mast; the luff runs the mast's full height. An Optimist is a spritsail.

They need four measurements where a triangle needs three, and we cut all of them: see classic & traditional.