Because pockets are sometimes hidden from view, it is difficult to know, from images alone, when pockets first became a standard part of an ensemble. I found a fascinating article about pockets on the Victoria and Albert Museum website, a valuable resource for seeing the shape and purpose of ladies' and men's pockets in the 19th century, the setting for my current work-in-progress. In addition, the website has illustrations as far back as the 17th century.
In the 18th century, pockets were underneath ladies' petticoats, as seen in photo at the right. Men's pockets were sewn into coat and breeches' linings, much as they are today.
Because there was less privacy in previous centuries, when families frequently shared rooms, people sometimes kept their personal possessions in their pockets.
Before handbags came into general use, pockets were used as a carryall, where ladies could carry common articles like thimbles or scissors, as well as money, snuff boxes, smelling salts, or even food and a bottle of gin.