What do the following words have in common?

SUM

FIRS

BEES

CRABS

PIANOS

PENGUINS

HINT:

EARL and DRACULA could be added to the list.

Answer:

Each word can be preceded by a "royal" noun or adjective:

PRINCELY SUM

NOBLE FIRS

QUEEN BEES

KING CRABS

GRAND PIANOS

EMPEROR PENGUINS

(DUKE OF EARL)

(COUNT DRACULA)