The lower fore-corner of a sail. To tack is to go about or shift from one tack to another. The side on which the wind blows on the sail, as starboard tack or port tack. This term probably originated with the square rig, as "port tacks" aboard means that the lower port corners of the sail are now hauled inboard, whereas when the wind was on the other side these corners had been hauled outboard by the sheets.