What is "attendant keys"

Attendant \At*tend"ant\, a. [F. attendant, p. pr. of attendre. See Attend, v. t.]

1. Being present, or in the train; accompanying; in waiting.

   From the attendant flotilla rang notes triumph.
   --Sir W. Scott.

   Cherub and Seraph . . . attendant on their Lord.
   --Milton.

2. Accompanying, connected with, or immediately following, as consequential; consequent; as, intemperance with all its attendant evils.

   The natural melancholy attendant upon his situation added to the gloom of the owner of the mansion.
   --Sir W. Scott.

3. (Law) Depending on, or owing duty or service to; as, the widow attendant to the heir.
   --Cowell.

   Attendant keys (Mus.), the keys or scales most nearly related to, or having most in common with, the principal key; those, namely, of its fifth above, or dominant, its fifth below (fourth above), or subdominant, and its relative minor or major.