Gambeson
A gambeson is a padded jacket used to protect the torso (and generally arms as well) from bruising blows. In period, the gambeson was a foundational garment worn under armour or on its own and would have been worn by thgose who could not afford better.
Gambesons in the SCA
In the SCA, many fighters do not wear gambesons or heavy armour, taking advantage of the SCA's rules to maximize their speed and mobility. When a gambeson is worn, it is generally worn under a heavy fighter's rigid armour, and for fencers it is generally worn just by itself with minimal added protection to groin and joints.
Ideally, a gambeson is be made of tough, heavy fabric on the outside, and soft, comfortable fabric on the inside, with a layer or two of quilting between them for padding.
