PC-TRAC is MicroSpeed's original full-sized, three-button trackball (the third button is Drag Lock). 100 percent Microsoft mouse compatible, PC-TRAC is available is two versions serial and PS/2 mouse port compatible. The low profile case of the PC-TRAC acts as a natural hand rest and minimizes the wrist angle by mirroring the natural contour of the hand. It's designed to easily accommodate both right and left hand users, as well as different size hands.??The Drag Lock feature of the PC-TRAC makes this product comfortable. No need to hold a button down to highlight or select an item. Simply hit the Drag Lock button and the button stays "locked" until hit a second time. Work with ease, and a more relaxed hand motion.