Control-C else, typically returning control to the user. Ctrl-C was one of a handful of keyboard sequences chosen by the program designers at Xerox PARC to control text editing, with Ctrl-Z (Undo), Ctrl-X (Cut), Ctrl-V (Paste), and Ctrl-P (Print). The first four letters are all located together at the left end of the bottom row of the standard QWERTY keyboard, and P towards the upper right. The equivalent key combination on Macintosh computers is Command-C. Control-C as an abort command was popularized by TOPS-20 and TOPS-10 and adopted by other systems including Unix. It was also adopted by Digital Equipment operating
PLUM keyboard non-letter characters. The extra Function keys F13, F14, and F15 map to Print Screen, Scroll Lock, and the Pause/Break key respectively, although no Scroll Lock light exists. There are also six blank keys surrounding the Arrow keys, which do nothing. It is debatable whether this layout reduces repetitive strain injury, but the general consensus is that while it might prolong it slightly, the QWERTY keyboard might simply aggravate pains without causing the disease, and studies have shown that typists are not any more likely to develop RSI than normal people. Users of PLUM keyboards, however have claimed that after switching