Autoharp
The autoharp is a many-stringed zither with chord buttons, each of which pushes down an arrangement of felt blocks that dampen all the strings except those in the chord. 15- and 21-chord autoharps are common.
Autoharps can be retuned easily with the aid of a square-head tuning lever (the same as used on harpsichords and hammer dulcimers). Careful planning may be advisable to make the best of the interdependence of the tuning of each chord, and experimentation is a viable option too.
See Henry Lowengard's page for some adventures in 5-limit 21-chord retuning.
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