The double bass (doublebass
or contrabass) gets its name from the fact that in classical music its
usual role is to double - one octave lower - the line of the cello which
plays the bass clef line in the orchestra string section. Since the
standard tuning has E as its lowest note and the cello has C as the
lowest note, different strategies have evolved to allow the double bass
to play those notes below the low E. None of these strategies is
perfect, each has advantages and disadvantages.
Joel Quarrington and some others have successfully adopted tuning in fifths (CGDA) an octave below the cello - great if you can learn the technique and are not worried about being able to play other peoples' instruments.
Five string basses have the notes on an extra string conveniently located next to the usual ones, but they often have bow clearance difficulties and an extra string adds additional tension and mass to the bridge and top which usually results in a slower response. (While I have never made a five string bass, I certainly could make a five string neck and bridge if a client wanted one).
A third option is the addition of an extension. This allows the use of a longer string in place of the E string, enough to get down to a low C or even to a low B. The extension notes can be fingered by reaching up over the scroll with a “gate” placed to stop the string at the E position and additional gates can be provided to stop the Eb, D and Db notes.
Yet another option is to provide a moveable gate which can be placed at any note from E on down. I have made several extensions for my own basses and others over the years. All of my basses are made with clearance provided so that if an extension is installed, the string can be routed over the wheel at the top and back to its original peg without the need to cut away any part of the scroll, a practice which I deplore. I believe that it is important to be able to tune each of the gates so I incorporate this capability into the design.
Here are some pictures of extensions on my basses.