4.18The 3rd and 7th are especially important

For seventh chords, two notes tell you most of the chord's harmonic character:

3rd

and:

7th

Take G7:

G B D F

Its 3rd:

B

Its 7th:

F

Those form a tritone.

When G7 resolves to C:

B → C

F → E

That is the essential resolution.

So you can reduce:

G7 → Cmaj7

to:

B–F → C–E

This is called guide-tone voice leading.

Very important for jazz.