Thirds are a lot more useful as double stops than seconds, and obviously they’re key building blocks of most chords. Luckily, no stretching or string skipping is required to grab major or minor thirds across the neck. They work nicely hammered on to/from fourths or from major seconds as well.
This may be the least useful individual diagram I ever post, but for the sake of completeness, I am starting my run through of intervals with major/minor 2nds. I worked through Truefire’s Interval Insights course, and I now feel even more confident that knowing where to grab all of the intervals on the neck is a worthwhile skill to have. I can’t demonstrate how to use them in context here, as Rich does, but it should be pretty simple to lay out the locations of each set to refer to for your own experiments.
Seconds are neither particularly consonant nor comfortable to grab. I kept stretches to a three fret minimum in this diagram, as going out of your way to grab either of these doesn’t seem necessary.