We have a wealth of archeological information on coats of plates from the remains of the Battle of Wisby in 1361. There is an excellent book called Armour from the Battle of Wisby 1361 by Bengt Thordeman and published by Chivalry Bookshelf which includes reconstructions based on the findings at the site. Online, there are pictures of some nice reconstructions based on the Wisby finds at this site.

The Wisby coats were categorized into four types based on the organization of the plates. The plate organization is this reconstruction of based on Wisby coat of plate Type II, which has two rows of vertical plates on the front and opens in the back. I constructed the cloth parts of the armour, while Magnus was responsible for the plates and their attachment to the garment.

It's thought that the coat of plates was the evolution of the armoured surcoat (and, that both coats of plates and lamellar armour have their roots in Eastern armours). Thordeman says "The earliest traces of iron armour below the surcoat know to me are to be found on the sculptured figure of St. Maurice in the Cathedral at Magdeburg of about 1250..." "...along the upper and lower edge of the girdle, and at the same height right across the front, there are two rows of large rivet-heads which can only be explained as the fastenings for iron plates on the inside of the covering. "(p. 285-6)

This Wikipedia and Wikimedia Commons image of St. Maurice at the Magdeburg Cathedral is from the user Chris 73 and is freely available at http://commons.wikimedia.org/wiki/Image:Saint_Maurice_Magdeburg.jpg under the creative commons cc-by-sa 2.5 license.

Above, is a picture of the sculpture of St. Maurice from the Magdeburg Cathedral. Along side of this is the inside view of a reconstruction of the St. Maurice armoured surcoat. Though this reconstruction lacks the integral mail hood that appears to be part of the surcoat on the statue, and is made with plastic plates instead of iron, it's relation to the coat of plates seems clear when comparing it to the reconstruction of the Wisby Type II coat of plates below (which itself is lacking a "belly-dip" where the center front plates extend lower than the side plates).

Though many of the metal plates that formed the rigid protection of this garment survive, there is far less archeological evidence to suggest exactly what the covering was made of. In regards to this, Thordeman states on page 210 that "Most of [the armour found in the Korsbetningen graves] is constructed in such a way that iron plates are attached to the inside of a covering or coat which, in those cases where it could be proved, was made of leather but may also have been made of, or covered with, cloth."

There was evidence of both cloth and leather having been used as covering materials on some of the Wisby examples. The author specifically mentions cloth in the examination of coat of plates number 2, type 1 on page 347-348: "there is...on the outside of these plates a coating of rust with distinct fabric pattern....These traces of cloth do not, however, show conclusively that the covering consisted only of cloth; they may also conceivably be the remnants of a cloth lining of a covering of some other material." And, in examination of another group of plates (from page 391) the author states "The covering of the armour was made of leather, which is clearly seen from the appearance of the coating on some of the plates."

That being said, I had some hesitation using leather as the covering for the coat of plates as I thought it might be a warmer garment than using linen or hemp. Though I suspect in period leather was more commonly used for a coat of plates as the garment is easier to construct of leather and it seems a sturdier material to rivet plates through, I constructed this coat of plates from two layers of heavy-weight linen, interlined with a third layer of the same material. The fabric has actually stood up quite well to the rigors of SCA heavy list combat.

The image to the right is a great example of a coat of plates under a surcoat from the Soldiers at the Holy Sepulchre from the Strasbourg Cathedral, c. 1340-1345. Note the visible rivets and the shape of the plates from the drape of the fabric.