Abstract: Several horizontal links have inward-projecting horizontal flanges for supporting the treads of a stairway. Such links are uniformly spaced apart elevationally by upright parallel links pivoted to the horizontal links to form a lazy tongs linkage. One such lazy tongs linkage is permanently attached in inclined attitude to the conventional weight-bearing supports for the stair treads, such as sidewalls or inclined stringers, at each side of the rough opening for the stairway. Prior to permanent installation, the lazy tongs linkage can be extended or contracted by conjoint swinging of the upright links to adjust the spacing of the horizontal tread support flanges so that the total rise of the linkage matches the total rise between adjacent floors as actually constructed. The support is designed to fit a range of expected total rises between the adjacent floors while maintaining a rise and run for each step within acceptable limits.