What about the different sizes and tunings of ukulele?

The Rainbow Ukulele books are written for GCEA tuning, with a high G (4th) string.

Rather confusingly, there are three sizes of ukulele (soprano, concert and tenor) which can all have the same tuning. Younger players will usually get on best with a soprano. Older players – especially those wishing to play advanced solo arrangements on ukulele – will often be more comfortable with a larger instrument.