The bottom rail is the slightly heavier slat or stabilizing piece attached to the bottom of a horizontal blind to keep it down. Hold-down brackets, when optionally used, attach to the bottom rail and the sill.
The bottom rail is the lowest slat on a horizontal blind or shade. It is often thicker than the other slats and/or weighted to provide proper anchoring of the window covering and ensure that it hangs straight. The bottom rail is usually color-coordinated to match the rest of the window covering.