The "Guitarre d'Amour" was an invention of Johann Georg Stauffer
in 1821. It was the size of a cello, had a fretted fingerboard, and six strings.
Think of it as a guitar/cello. You held it and tuned it the same way as the
guitar, with the strings in order E, A, D, G, B, and E, and played it with a
bow like a cello.
It was later known as the Arpeggione, because of it's facility
with guitar-like arpeggiations.
It never became pouplar, however, and became obsolete because it was awkward
to hold with its guitar-shaped body, and because of the low curvature imposed
on the strings, it was hard to play a single note on a string very loudly.
There is probably only one significant work written for the Arpeggione, and
that was Franz Schubert's Sonata in A Minor "Per Arpeggione" in 1824,
in which Schubert made extensive use of the arpeggionating abilities of the
instrument, and the large range of the instrument. That piece is now taken over
by cellists or violists.