Dom7 Shell

Root + 3rd + ♭7

Prove it — just do the practice exercise.
C7 Shell Voicing
C (R)E (3)Bb (♭7)
In the dominant shell, only the seventh changes:

Root
Major 3rd — same as Maj7
Minor 7th — one semitone lower than Maj7

Example: C7 Shell = C + E + Bb

Remember: The 3rd stays the same, only the 7th moves.