The original theory of quartering upon which all rules are based is that after a marriage with an heiress, necessitating for the children the combination of two coats, the shield is divided into four quarters. These four are then numbered from the top left-hand (the dexter) corner (No. 1) across towards the sinister corner (No. 2) side of the shield; then the next row is numbered in the same way (Nos. 3 and 4). This rule as to the method of numbering holds good for any number of quarterings.
A Complete Guide to Heraldry by A. C. Fox-Davies, p. 547