A master clock was the controlling center of IBM's Self-Regulating Electric Time System, and it was the source of accurate time - up to within 10 seconds a month of correct time - for the various secondary time pieces, such as indicating clocks, attendance and job cost recorders, distributed throughout the system. The "Grandfather" style clocks, either No. 21 (spring-driven) or No. 31 (weight-driven), beat 60 times a minute, possessed self-winding movements and came finished in either red or brown mahogany. The clocks were marketed in the 1930s by the International Time Recording Company, an IBM division.