mjwjr, if your
slide rule is set to F and the insert is the Am form, you're playing an Fm. Whatever the letter is in the cut-out window has the flavour of the form. So if it says 'E7th form' and it's set to 'C', you're looking at a C7th ... the shape is that of an E7th, barred.
Shapes -- the way the notes arrange themselves on the fretboard -- are mappable and memorable.
(The slide rule referred to is the $6 downloadable Chord Finder mentioned on the home page of Guitar for Beginners)