A skew adjustment method and apparatus for adjusting the skewed condition of a movable rail of a window covering in which the movable rail is supported by lift cords wrapped around spools. A drive train disconnecting mechanism is used to disconnect a first spool from a second spool, the first spool is rotated relative to the second spool, thereby changing the length of the first lift cord relative to the second lift cord until the movable rail is in the desired position; and then the first and second spools are reconnected so the first and second spools rotate together. An end cap may be used to enable multiple disassemblies and reassemblies of the end cap to access the skew adjustment mechanism. In one embodiment, there are two movable rails, and the lift cords for both movable rails extend through the same rout holes in the covering. Disconnecting the first spool from the second spool may activate a mechanism that prevents the second spool from rotating.