Two Planes have Line in Common

Theorem
Two distinct planes have exactly one (straight) line in common.

Proof
Take two distinct lines in plane $1$.

From Propositions of Incidence: Plane and Line, they each meet plane $2$ in one point each, say at $A$ and $B$.

Thus $A$ and $B$ both lie in both planes.

Thus the line defined by $A$ and $B$ lies in both planes.