Formerly, as has been seen, a number of small vessels were classed as frigates. About the year 1775 they were placed in a different rate, and those carrying 20 guns had now the name of 20-gun post-ships given to them, signifying that they were commanded by post-captains. Afterwards vessels still called frigates, carrying 24 guns, were also ranked as post-ships. The French called vessels of this size corvettes, from the Italian word corvettore, to leap or bound, from which we have derived the word curvet. The French afterwards applied the name to ships of 24 guns. In order to mount all these guns on a single tier, it was necessary to increase the dimensions of the ship, and thus she could carry heavier metal than those ships mounting their guns on a quarter-deck and forecastle. The English, following their example, afterwards called all ships carrying 24, 22, and 20 guns post-ships, and those carrying 18, 16, and 14, or any less number, ship-sloops, to which the general term of corvette was afterwards applied. The English did not apply the term corvette to brigs, but designated such two-masted vessels as brigs-of-war, though they are sometimes spoken of as brig-sloops. It will thus be understood that a ship that mounts 24 guns at least on a single-deck, and other guns on a quarter-deck and forecastle, is properly called a frigate. When, however, the waist is decked over and has raised bulwarks with ports in them filled with guns, the vessel becomes a two-decked ship. It is necessary to explain the term "flush." In sea language it means level, a flush-deck is consequently a level deck extending fore and aft. Such are all the decks of a man-of-war, except of the upper ones. Many merchantmen are also built in the same way, but others rise abruptly a foot, or two or three feet, towards the stern, the higher part of the deck becoming the quarter-deck. Ships thus built are spoken of as deep-waisted, because the centre part is deeper or lower than the after-part. The bulwarks in the same way sink in proportion at the break of the quarter-deck. Up to the present day many of the largest ships-of-war are flush-decked, as are all brigs-of-war and many corvettes, but a frigate, which must have a quarter-deck and forecastle, cannot properly be said to be flush-decked, although, in fact, the gratings or gangway at the waist give her the appearance of being so to the unsophisticated eye.