The Registers

On the left,the row of jacks operated by the upper keyboard, voiced lightly (that is, the quills cut to sound evenly, with the same finger pressure needed for each key), so that this register is the quietist. The middle row is the 4' - which plucks strings tuned to an octave higher then its neighbours. This provides an added richness of sound when plucked with one of the (normal) 8' registers. Its strings are half the length, and only reach halfway down the soundboard. Strings only sound between the nuts (left of the picture) and the bridge (which you can see on the soundboard pictures); the bridge transmits vibrations from the string to the soundboard. The third register, to the right, is operated by the lower keyboard, as is the 4'. They can be played together. You can see the brass lever at the bottom of the photo - it controls whether the plectrum plucks or misses the key when the keyboard pushes the jack upwards. All three registers are controlled by levers, and die ways movement can be adjusted by the little brass capstans that can be seen in their ends. The upper keyboard can be coupled to the lower by pushing it forwards and backwards; so both 8' registers can be played at the same time, and the 4' register can be added so that three sets of jacks are plucking three sets of strings with one press of a key.
The upper register, plucking closer to the nut, makes a more nasal sound than the lower; it's the same on the guitar.