When mariner types start telling tales, the fish seem to get bigger and the truth quotient seems to get smaller. So we weren't surprised when we heard from a fellow trying to ascertain if it's true that mainsheet refers not to a sail (the extent of fabric by means of which wind is used to propel a ship through the water), but to the rope or line by which a mainsail is trimmed and secured. (来源：http://www.EnglishCN.com)

Indeed it is true: sheet—from an Old English word meaning "lower corner of a sail"— actually refers to the rope or chain that regulates the angle at which a sail is set in relation to the wind.

Then why are sails (those extents of fabric featured in the propelling of a ship through water) also called sheets? Don't pin it on sailors seeking an easy way to confuse landlubbers. In fact, English contains not one but two words called sheet.

The sheet referring to the rope or chain (or line, to use the proper shipboard term) that we mentioned earlier took a different path into our lexicon than the sheet that names a "sail, a usually rectangular piece of paper," or "a flat baking pan of tinned metal." The Old English ancestor of this sheet was akin to the Old English ancestor of the other sheet, but the sail sheet and the rope sheet entered English at two different times from two different directions.