THE CHEST PIECE

MATERIALS
The canvas front can comprise of various materials ranging from haircloth, felt, domettem and others. These materials ultimately effect how the garment feels, sits and the shape it takes on. Rarely used Camel Hair is apparently better then horsehair for its softer hand, making it more comfortable but its price tag often results in the fabric being constructed from a blend. Horsehair in itself can cost a fortune varying on the amount used and a fully canvased suit using horsehair can cost up to $80,000, therefore is also often blended. Horsehair is great for the properties it holds, it will shape to the contours of your chest after a few wears, creating a three-dimensional shape. In a blend horsehair is inserted into the weft picks, which are then counted per inch, and determines its quality and resilience.
Today, many suit manufacturers use cotton blend or synthetic canvasses, these can be made of such fibres as Rayon & Viscose which ultimately lower their costs of production.

CONSTRUCTION
There are many ways to construct the canvas, depending of the desired result. Suffice it to say that all layers are attached together using either machine or hand padding these can then be soaked in water overnight, then hung to dry to pre-shrink before being basted into the coat.
It is also important how these are attached; chest pieces can be fused on or tacked in, by tacking pieces into position this allows for slightly more mobility thus better moulding. Within the chest there will be one, two or three layers of felt, wool, cotton or horsehair in some combination. A standard English suit will have a layer of horsehair running all the way behind the forepart of the suit, with a layer of felt in the upper half. Those two are sewn loosely together with basting stitches and then attached, again loosely. Italians are known for creating less structured suits due to the warmer conditions therefore may consist of fewer layers.

http://stylecrave.com/2010-05-10/how-a-suit-is-canvassed/