Maybe not considered by an instrument's owner all that often (until it fails!), but glue is probably the most important ingredient of all in the construction of a musical instrument. Down through the history of lutherie, and just about anything else made of wood, animal glue HIDE GLUE has proven itself to be a very reliable adhesive under most normal conditions, with the added benefit of being able to be steamed apart relatively easily for any future repairwork.
From the time I discovered shellac as a finishing material, I also discovered hide glue and the benefits of its properties for instrument making. It is used on all instruments made here in the shop for all the major joints, and as it dries back to its hardened crystalline state, I feel it benefits the transfer of energy around the instrument and helps, albeit subtly, with the clarity and quality of the response of an instrument. It is prepared by soaking in water for 30 minutes and heated in a temperature controlled glue pot for maximum strength and a workable viscosity.