One of my exercises, in learning to tune by ear, is to tune the 1st course to the 6th course (two octaves).
At 5 cents of error, the beat rate is so slow that there are only one or two beats before the 1st course dies away. Also, the first course is very sensitive to movement of the peg. So it is not easy to consistently get 0 cents of error. This morning I averaged 1.4 cents over 10 tries. Elimination of sympathetic vibration helps. To eliminate all the extra Gs, I mute one string of the 6th G course, and both strings of the 5th C course (the 2nd harmonic of C is G). Also, the 2nd harmonic of G is D, so you could also mute the D courses. Mutes can be made of wedges cut from clothespins, rubber bands, and glue.
