newbie question

I'm having difficulty creating planes. I choose "Through Lines/Points" and
can only select one line. Why won't SW let me select additional lines to
define the plane?

TIA.

 

Because it could overdefine the plane. Think about it - a line defines 2 of
the 3 points necessary to define a plane, and then all you need is another
point. So, pick your line, and then pick the endpoint/midpoint of the other
line.

WT