N8N wrote:
Clamps are required. The knockout holes require separate clamps. The
reason those holes are used is to allow connection of conduit as well
as Romex and BX type cables. (you simply use a different clamp/
connector for each type of wiring.) If you buy boxes with integral
clamps make sure you get the ones for Romex not BX (they look similar
but are subtly different.)

For Romex, you can also use the little black Arlington buttons instead
of clamps

Thanks for the info. What about the little plastic (often blue in color)
outlet boxes? They have rectangular knockouts on the back end; is there a
clamping fixture for them, or does the wire just hang freely?

Jon