A bend is also a term in heraldry. It refers to a colored band that runs from the upper right (as seen by the viewer) corner of the shield to the lower left, covering approximately one-fifth the width of the shield.
A bend sinister is a bend which runs from the upper left (as seen by the viewer) corner of the shield to the lower right.
In bend refers to the appearance of several items on the shield being lined up in the direction of a bend. For example, if three or four of a particular item were lined up so as to run from the upper right to the lower left.
Party per bend refers to a single line which runs from the upper right to the lower left, dividing the shield into two halves.
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