Grace

From MiddleWiki

The respectful salutation made to someone of ducal rank, e.g. a duke or a duchess.

One would address a duke or duchess as 'Your Grace."

One would refer to a duke or duchess as 'His Grace' or 'Her Grace.'

It is also acceptable, though little used, to refer to a king or queen as, for example, 'Your Royal Grace.'

Dukes and duchesses can be recognized by their ducal coronets ornamented with strawberry leaves.