Bed rail that tucks under mattress

A bed rail for engagement between a mattress and a box spring and having a base support and a guard frame, with guard frame having a pivot that pivots and slides in the base support. The base support includes a first seat that seats the pivot and a second seat that seats a biased piece sliding axially on a first support member of the guard frame. The first support member extends from a plastic piece that forms a junction from which a second support member and the pivot extend and that nestles up to the second seat.