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.
Diving back to triads in all positions. I don’t naturally grab these triads up the neck, but now is as good a time as any to start.
I didn’t realize just how useful learning all of these triad positions would be until they started clicking, particularly up and down between the three core shapes in a given key (root on string 2, 3, or 4). Beginning with the shape below as the I chord gives you a ton of options nearby. The ii, IV, and vi have multiple options that are easy to grab or switch between.
With a bassline or second harmony covering the low end, you can easily cycle through the upper extensions of the underlying chord (upper structure triads once you’re beyond R,3,5 if I recall correctly). Throw sus2 and sus4 in and your palette extends even further. I should probably record an audio example to demonstrate.
The best thing to me is that you can always map a diatonic mode back to a home Ionean key, so just knowing which I chord you refer back to means you can grab extensions up and down the neck. For instance, A Aeolian is just the sixth mode of C Major, so all of the triads will be the same as those in C. Even grabbing the C Maj triad just gives you an Am7 over an A bass note.
Here’s a link to a tool I run from my personal page. It allows you to pick a home key (Major/Ionean) and see everything I could come up with that qualifies as an interesting substitution or borrowed chord. I’m sure there will be more to add down the line, but I’ve barely scratched the surface of putting these into practice.
On to strings 2/3/4. No shortage of options here either. For the rootless versions, it’s easiest helpful to think about placing the minor 7th position relative to the target root.