If the two strings in a course are too far apart, your finger will jam between them when playing, completely messing up the tone and intonation. I recently learned this by trial and error when completing a long scale mandolin. On my third try, I ended up with the course strings about 1/8" apart (center to center). Courses were approx. 1/4" apart (edge to edge). These dimensions are not exact, and your results may vary. With close spacing, I was concerned about strings buzzing against each other; but that worry paled against my fingers jamming between the strings with wide spacing. There is a similar issue at the bridge, but you can get away with a little wider spacing at the saddle, maybe 3/16". Again, results vary, and be prepared to make more than one nut or saddle before pronouncing the action set. The courses in the attached photo are too far apart. Good Luck!! Steven (CarolinaFiddle.com) P.S. A "course" is more than one string installed close together and fingered in unison by the player. Most mandolins have 4 double courses. You will rarely also see triple courses on some instruments.