What do they call the power supply cable multipliers?

I once got my hands on a multiplier cable that turns one power supply
cable into two. This cord plugs into the end of one of the power
cables that plug into the power supply and you get two cable ends to
use. Computer power supplies never have enough cables. I need to get
some of these things, but I am not sure of the name for them, so when
I call, no one understands what I want. I'd call them a 3-way, like
the 3 ways used for outlets, but I'm sure thats not the correct name.
.

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