stim is absolutely right, using the thumb the melody can be picked, it is easiest in c or g, but apparantly can also be done in aed, though i have never tried it in these keys.it is very usefulk inthis style toshow the pupil the major key of c and g, which is generally a string lower
the same principle can be use in open chord tunings such as dadf#ad, dgdgbd.
the other form of melody picking is sometimes known as piedmont style , used by john hurt, etta baker,and mance lipscomb it works well in c and g too, but the idea is to pick the melody with the fingers[generally]though in some styles of this the thumb can play third string melody, sometimes third string melody is plated by index[up]