Trews are narrow, tight-fitting trousers, often in Tartan, worn by Scotsmen when not kilted, and are part of the mess uniform for many Scottish regiments.
A kilt is a skirt-like garment which, in past times, was worn by men in the 17th and 18th centuries in the Scottish Highlands. Since that time, it has come to be one of the traditional symbols of all of Scotland, being worn by men in modern times as a celebration of their Scottish heritage. It is also worn by women and girls as part of the costume of pipers and Highland dancers.
The garment itself consists of a length of woolen fabric called tartan which is wrapped around the waist, fastened with buckles and straps, and hangs down to the knees. It is pleated around the sides and back and the fabric displays a plaid pattern which, in modern times, has come to be associated with a Scottish clan.
Historically, the kilt had its origin in the Irish costume of the 16th century which consisted of a leine (a type of long shirt) which was worn with a mantle or plaid, the latter being a length of fabric cast about the shoulders. Sometime in the late 16th century, the mantle began to be worn belted, with the portion hanging below the waist first being loosely gathered into rough pleats. The mantle, worn in this manner, has come to be referred to as the Great kilt or belted plaid.
By early in the 18th century (approximately), the style developed of wearing only the lower portion of the belted plaid, again loosely gathered and belted about the waist. The resulting garment is now referred to as the little kilt.
The modern form of the kilt finally began to emerge in the late 18th century with the Regimental kilts, especially that of the Black Watch. At that time, the pleats (actually, a loose gathering of the fabric) were stitched down to facilitate putting on and wearing the garment. The garment used at that time was box pleated. The knife pleats featured on the great majority of modern kilts were adopted only beginning in the mid-19th century.