Nikolai Rimsky-Korsakov · section 52 of 96
Indirect means
Read and hear this section in the playable edition →§ 50. — To modulate well from a minor key to the key of the subdominant (a–d), one first modulates to the major key relative to the following key: a (F) d.

The reason for this modulation is the contradiction between the minor tonic chord of the key of a and the major dominant chord of the key of d.
To modulate from a major key to the major key of the subdominant (C–F), one can also modulate indirectly: C (d) F.

Likewise, to pass to the key of the II degree (C–d), one modulates indirectly:

REMARK. — In all the examples above, the six-four chord in parentheses is only the formula of the cadence of the second kind, and, being of only relative importance, it may be left out without harm.