Fleur-de-lis

The fleur-de-lis is a symbol worn or displayed by certain characters related to the Storm King in the Girl Genius universe. It is apparently the sigil of the House of the Storm King.

As worn by real characters
The fleur-de-lis is worn by a group of clanks that at the gates of Mechanicsburg with the stated intention to rescue Agatha in the name of the Storm King.

Less elaborate versions of the insignia are worn by Van Rijn's Muses. and 's insignia have a  (only the top part of the fleur-de-lis, and the lines are more flowing). version is slightly more elaborate. Note that Van Rijn created the Muses for the Storm King, so it seems reasonable that they carry a (version of the) Storm King insignia.

As worn by fictional characters
The fleur-de-lis is worn by the King, troops, Prince, Nurse, and burnt-out castle in Klaus's about how the Storm King became King.

In the Storm King Opera, only the muses, but not the Storm King himself, are as wearing the fleur-de-lis.

As not worn by Tarvek
Although Tarvek is the present-day Storm King (via his mother's side), he has not been seen wearing the Storm King insignia. Rather, he used to wear the Sturmvoraus insignia (via his father's side), most recently. This makes sense, given that Tarvek's Storm King ancestry was at that time a closely guarded secret. At present, Tarvek does not wear any insignia. Indeed he has not worn any formal clothing, or too much clothing at all, for a.

Outside world
In the real world, the fleur-de-lis is a symbol frequently used in heraldry, and often associated with France.