Dimidiation

In heraldry, dimidiation is a method of marshalling (heraldically combining) two coats of arms.

Whereas impalement involves placing the whole of both coats of arms side by side in the same shield, dimidiation involves placing the dexter half of one coat of arms alongside the sinister half of the other.

In order to avoid these drawbacks, it became customary to use more than half of each coat of arms when combining them through dimidiation.

Once this practice had begun, the logical progression was to include the whole of both coats of arms in the new shield, so that in effect, impalement replaced dimidiation as a method of combining coats of arms.

A general rule which carries over from dimidiation to impalement is that if a coat of arms with a bordure (or tressure, orle, etc.)

Coat of arms of Hastings , showing a partially dimidiated shield, where the front halves of the upper and lower lions are joined to the rear halves of ships. This motif features widely in the heraldry of the Cinque Ports .
Horizontally dimidiated rabbit and fish, arms of the town and gmina of Prochowice , SW Poland. Horizontal dimidiations are rare, if not unknown, in English heraldry.
Coat of arms of the consolidated commune of Les Andelys , dimidiated (French: Mi-parti ) per pale, three droops of grapes and three castle keeps. Dimidiations are rare in French heraldry.