The canton is a square section of the shield, issuant from one of the upper corners. It’s one of the earliest of charges, found in the arms of John of Brittany c.1285 [Asp2 213]. Unless stated otherwise, it issues from the dexter chief corner; a “sinister canton” issues from the sinister chief.
The canton is a peripheral ordinary, though classed as a sub-ordinary by some authors. The same charge is also called a “quarter”; 19th Century writers tried to distinguish between the two, saying the quarter was one-fourth the area of the shield, and the canton one-ninth. No such distinction is made in medieval or Society heraldry; it’s drawn as large as necessary, to accommodate the design on the shield.
Charged cantons were a period form of augmentation, and their use is so reserved in the Society.
Alan Fairfax bears: Bendy sinister Or and gules, a canton sable.
Thomas of Red Square bears: Argent, a quarter gules.
Helena d’Évreux bears: Per fess Or and azure, a sinister canton azure.