See also: Sail in Pictorial Dictionary of Heraldry.
A piece of cloth hung to provide propulsion for a ship.
Default orientation: billowing, hanging from a horizontal yard. No proper coloration.
Sail Hanging from its Yardarm (1) §
Source: Insignia Nobilium Veronensium, Vicentinorum. Artist unknown. (From the arms of Vella, folio 109.)