In early medieval times, ‘the staple’ meant England’s staple export: wool. But it was inconvenient and inefficient for the king’s men to collect the customs duties that were payable on the exported wool from every one of the hundreds of little English ports all around the country. London, Bristol, Ipswich and Sandwich were major ports but little ships could sail from any small harbour or river estuary. Therefore, since wherever the ships had sailed from, they were all taking their cargo of wool to Flanders (modern day Belgium and north-east France), it was easier to collect the customs when they arrived at their destination. In 1313, Edward II ordained that all merchants had to land their ‘staple’ at a port he would designate. During the Hundred Years War, England acquired Calais from the French and from the mid-fifteenth century until 1558 this port became the convenient Calais Staple, where customs duties were collected on all English wool exports.
From “A Year in the Life of Medieval England” by Toni Mount.
The image is Old bird’s-eye view plan of Calais by Braun & Hogenberg 1597