In June 1937 he was again recorded in Dallas, and although he was reported to be playing a seven string, guitar by this time, it wasn't used on the sessions.

How many strings did Robert Johnson's guitar have?

This Open-Eb tuning is the same sequence of harmonics, except it starts with an Eb, and because Robert Johnson has 6 strings instead of Keith's five, the next harmonic in the series is the 7th harmonic.

Was Robert Johnson bad at guitar?

Late in life, House remembered Johnson as a "little boy" who was a competent harmonica player but an embarrassingly bad guitarist.