Some 19th century guitars had more than 6 strings in order to improve the instrument's range. Usually, these extra strings were bass strings tuned in pitch below the guitar's low E string. This was not a surprising development, since the first 6-string guitar was simply a Baroque 5-string guitar fitted with an extra 6th bass string, that for whatever reason became the standard. There was tremendous experimentation with guitar design in the 19th century as evidenced by the wide array of body shapes, designs, and decorations, as well as the number of strings.
The extra strings were attached either to the side of the headstock, or by using a double neck. The construction must be slightly altered to widen the bridge, and also to handle the extra tension of additional strings. Unfortunately, if the bridge is too tight by over-compensation, the tone quality of the guitar will suffer, or sound different from a 6-string. A high quality multi-bass guitar with a great 6-string sound requires a skilled luthier to balance the tension requirements with tone production.
Some 19th century guitar music has the extra bass notes written in the score. Usually, they are written at normal pitch to accommodate the vast majority of 6-string players, but an octave adjustment (8va) is indicated. A great deal of Mertz is written with extra bass notes; they are written at 6-string pitch with an octave notation so that players with a multibass instrument can play the note at its intended lower pitch. Legnani has some pieces for guitars with 6 or 8 strings. Napoleon Coste's music usually calls for the drop-D 7th string; it is often played on 6-string guitars, but with compromises: Coste's music cannot be played in drop D tuning because it calls for 6=E relationships, and the Drop D is extensively utilized with great effect. It is of course possible for the modern player to raise the octave of these low notes to fit a 6-string guitar, but the full intended sonority of the music is somewhat compromised. The lower bass notes are not used all that frequently, but when they are written, they add a great deal to the piece.
The extra bass notes also provide sympathetic vibration with other strings, even on 6-string music. For example, if you play a D chord using only the 4 treble strings on a 7 or 8 string guitar, and then immediately dampen the strings, you will notice the low D string is vibrating at about 20% of plucked volume, even though it was not plucked. It vibrates sympathetically because it is the same pitch at a different octave. This provides a "pedal tone", a kind of background harmony that increases the resonance and overtones of the guitar, giving an interesting effect. This was not a new idea, as other ancient instruments used this principle.
A disadvantage of these instruments is the increased difficulty of playing. On a 6-string guitar, the 6th E string is always on top, thus providing an absolute point of reference. The 5th A string is likewise found as the 2nd string down from the top. On an 8-string guitar, the 6th string is now the 3rd string from the top, which takes away the top-side point of reference; the 6th string point of reference then becomes relative to the thumb's current location. The A string is found either relative to the thumb's current location, or relative to the trebles. 10 string guitars can be even more daunting at first. It can take several weeks of pure frustration and a great loss of accuracy to learn a multi-bass guitar. Once learned, however, accuracy and skill will improve over several months to full proficiency.