Some electric guitars have an apparatus called a string lock or lock down nut; this is a metal clip that screws into the guitar with allen wrenches to lock down all six strings at the nut of the guitar. The locking nut keeps strings from going out of tune when using a whammy-bar, a device that dramatically loosens all the guitar strings either up or down to create dramatic vibrato or dive bomb effects. If you try to tune the guitar with the pegs while a locking nut is clamped on the strings, you will break a string. There is a wonderful all purpose tool called a Guitool (price about $18) that has a string cutter, allen wrenches used by most guitars for locking nuts, and phillips head and slotted screwdrivers.
