Canton is a charge placed in the upper dexter corner. It is classed by some heraldic writers as one of the honorable ordinaries; but, strictly speaking, it is a diminutive of the Quarter, being two-thirds the area of that ordinary. However, in the roll of Henry III the quarter appears in several coats which in later rolls are blazoned as cantons. The canton, like the quarter, is an early bearing, and is always shown with straight lines.
The highly unusual usage of a "canton bendwise sinister" appears on the emblem of the 410 Flight Test Squadron of the United States Air Force.http://www.afhra.af.mil/factsheets/factsheet.asp?id=11998
The diminutive of the canton is the chequer that forms part of the field of chequy but cannot be shown as a charge on its own.
A canton sinister is a canton placed on the sinister side of the shield.
A plain, uncharged canton (sometimes a canton voided is also used this way) can be used as a mark of distinction, that is, not a mark of peculiar honour, but a mark making distinct that the bearer is a stranger in blood, e.g. a groom, not descended from the bride's family, adopting the bride's last name after the marriage, might upon receipt of a Royal Licence permitting this, use the bride's family's coat of arms with an uncharged canton or canton voided.