Parts of a sail
Tack
The forward bottom corner — where the sail is pinned down.
DutchHalshoekour Dutch loft →
SpanishPuño de amuraour Spanish loft →
The tack is the lower forward corner: on a mainsail it sits at the gooseneck where mast meets boom, on a headsail at the stemhead or furler drum, and on an asymmetric spinnaker at the bowsprit or stemhead fitting.
It is the sail's anchor point. Almost every tuning adjustment you make — halyard, cunningham, outhaul — is measured against the tack staying put, which is why it is another heavily reinforced corner.
Confusingly, "tack" is also the verb for turning the bow through the wind, and the noun for which side the wind is on (port or starboard tack). Same word, three meanings, and context does all the work.