Co-premise

A co-premise is a premise in reasoning and informal logic which is not the main supporting reason for a contention or a lemma, but is logically necessary to ensure the validity of an argument.

One premise by itself, or a group of co-premises can form a reason.

But this by itself does not guarantee a valid argument, see the fallacy of the undistributed middle for an example of this.

In this argument map of a simple argument the two reasons for the main contention are co-premises and not separate reasons for believing the contention to be true.

They are both necessary to ensure that the argument as a whole retains logical validity.