By default, an ukulele is tuned in reentrant mode, that is, the 4th string does not sound lower than the 3rd, but a fifth higher. The lowest tone is therefore on the 3rd string.

To extend the range (possibly also for reasons of aversion to retrograde), the 4th string can be replaced by a special string that sounds one octave lower than usual. This is no longer a fifth, but a fourth below the 3rd string.

In order to distinguish between reentrant and non-reentrant tunings, we speak, in the case of the popular C6 tuning, of high G for reentrant and of low G for linear tuning, because the affected fourth string is the G string.