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Webster 1913 Edition

Adj.

Of or pertaining to an archduke or archduchy.

Adj.

[See Archduke.] Pertaining to an archduke.

archducal ‎(not comparable)

Pertaining to an archduke or archduchess.
- 2012, Christopher Clark, The Sleepwalkers, Penguin 2013, p. 369:
  The archducal couple were strikingly unconcerned about their own safety.

pertaining to archdukes