A square fore-and-aft sail set on the foremast, but behind it, on a schooner or other similar vessel.
The lowest (and usually the largest) square sail hung on the foremast
A triangular sail set forward of the foremast: forestaysail.
The mast below the topgallant mast in a square-rigged ship.
The highest mast in a fore-and-aft-rigged ship.