Real chorded zithers have the little problem that they often have a good collection of chords, but then some fiddler or guitarist pops by and plays a little riff in E, or throws on a capo, and now they are in some other inaccessable key that you don't have chords for. That's very frustrating! PolyHarp has a virtual Capo feature that lets you transpose all the chords (and therefore, the strings) with the tap of a "+" or "-" button:
At the top is the current Base Key, the frequency that all the string pitches are based on. You can just touch it to bring up the frequency view.
Below that are the "-" and "+" buttons, which will transpose the Base Key by the interval specified below it. It's usually set to "V", and if you set that to I#, or 1:12, or 100 (cents), you can transpose by a semitone. If you are working with just intervals, you might want to set that to "3/2", and if you are working with some other equal temperament, like 19EDO, then you can set it to "1:19" or whatever is appropriate.
Note that you can also set chord buttons to "relativize" your chords, so tapping it will retune the strings on the fly!
Transposing will cause the frequency of the base tone to change, and in changing, it may creep into octaves that are higher or lower than make sense in your PolyHarp. The "=" button transposes the interval into the "third" octave.
However, some chord types are set up to not repeat (!) or repeat on intervals that are not octaves, and so which octave the base frequency is matters when using those chords.