Dominant Seventh Chords Theory Things   William Wieland
C7 & F7 7th "lower"

G7 same

A7, D7, E7 3rd "higher"

B♭7 double-stuffed oreo
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Examples:  Note Names   On the Staff
Practice with Chord Construction ( Click on the gear in the upper right to customize as follows:
Clefs Treble & Bass, Key Signatures only the top one, Chords only Dominant 7th, Difficulty Level 2.