The gambeson is a quilted doublet. It used to be worn under the armor and covered the trunk, arms and partially also the legs. Protected from infections that could create the wounds caused by the rubbing of the mail
It began to be used at the end of the tenth century as we know it but before there were garments with a similar purpose, such as the Roman subarmalis, for example.
For ordinary soldiers who could not afford chain mail, the gambeson was their only protection in battle, along with the helmet. Its decline was matched to that of the armor due to the appearance of firearms. It was no longer used in battle but as a piece of clothing