Another term for "expander" is "module", basically a key-less version of a keyboard (if that makes sense). Thus, the X4 is an expander version of the X1. Early midi keyboards had such limited polyphony that an expander version was often added to double polyphony without requiring a second keyboard. Nowadays, polyphony is less of a problem, so modules are used less for expanding a similar keyboard as to provide a new capabilities to a setup that has a different keyboard.

HTH

[This message has been edited by Clif Anderson (edited 03-18-2001).]