Two Points Usually Determine a Line

Two points usually determine a straight line, as only one line can pass through them both.

The exception is when the points are not distinct, ie. both points are occupying the same location in space. If this were the case, then the two points wouldn't define a singular line, but rather an infinite amount of line, because the two points can essentially be seen as one as they occupy the exact same space.

Also, when you're dealing with proofs, you can draw extra lines to prove your point. So in the reasons section of a two column proof, you would put "Draw Line" whatever the line/segment is and then put "two points determine a line" in the Reason side of the proof.