Abstract: An upholstered article of furniture (20) comprises a frame (26), assorted layers of padding and fabric that form upholstery (27), a backrest (29), armrests (31), cushions (33), and a seating module (10). The seating module (10) comprises a front rail (12), a back rail (14), a spring assembly (16) mounted upon the rails (12, 14), and a covering (18). The seating module (10) may be removed from and installed into a frame (26) of an article of furniture (20) as a complete unit by means of screws or other suitable attaching structure. Seating modules of different seating firmness may be interchanged (based upon consumer comfort preferences and medicinal requirements). The pitch (or tilt) of the seating module can be varied by changing the relative heights of the front rail (12) and back rail (14). For the case of sofas, the seating module (10) can be sized to be interchangeable with standard sleeper mechanisms.