Abstract: For adjustably locking relatively-sliding double closures, such as double-hung sliding sash windows, inserted between the adjacent frame and one of the sashes, this adjustable locking device includes a pair of interconnected first and second elongated bars, the first of which is T-shaped in cross-section and slidably movable in the channel-shaped second bar. One end of the first bar carries a pivoted screw upon which is threaded an adjusting collar, whereas its upper end carries a slidably mounted spring-pressed locking pin selectively engageable with one of a series of locking holes in the second bar. This pin is moved into and out of its selected locking hole by a thumb lever mounted on the first bar and having a pivotal connection therewith. A transverse attachment plate is secured to the upper edge of the upper bar for engagement with the upper channel of the window frame.