A flaunch, in heraldry, is regarded as an ordinary or subordinary, one of two (as the flaunch is never borne singly) semicircles protruding into the field from the sides of the shield. While supposedly the diminutives of flaunches are flasques and voiders (which likewise cannot be borne singly), these do not exist in modern heraldry.
Flaunches may be modified by the lines of partition.
Some heraldic writers say that it is a rule that the illegitimate son of a noblewoman must bear his arms upon "a surcoat"; that is, in the area between what appear to be flaunches significantly larger than normal. However, this is not really a rule, and has happened in one case at the most.
James Parker says that square flanks (flaunches) occur in the arms of Moselton.http://www.heraldsnet.org/saitou/parker/Jpglossf.htm#Fylfot