The Quadrangularis Reversum is a percussion instrument invented and built by Harry Partch in 1965. The instrument features an inverted diamond shaped centre compared to an earlier instrument made by Partch, the Diamond Marimba. Partch became aware of this possibility when it was pointed out to him by Erv Wilson who also helped him build the instrument.
The Quadrangularis Reversum inverted the key layout of the Diamond Marimba with sets of alto-range auxiliary keys on either side. Underneath the bars there are bamboo resonators. The instrument is made out of a eucalyptus branch and redwood structure and is one of Partch's bigger instrument sculptures. Partch used the instrument in his two last compositions.