The Model Ms made for the IBM RT PC and RS/6000 systems were variants of the IBM Enhanced Keyboard featuring an integrated speaker on the underside of the keyboard. The speaker, which occupies a space usually left unfilled on other Enhanced Keyboards, is believed to act as an audible cue when keys as pressed. The speaker is operated by the host system via extra pins in the unique AMP-made connector (RT PC) or the unused pins in a PS/2 plug (RS/6000). For PS/2 variants, the keyboard itself can still be used with standard AT or PS/2 class PCs, however, the speaker cannot be utilised by them without creating an additional circuit to signal the usually unused pins.