For most kinds of music, major and minor chords are used extensively. But sometimes you don't want a "normal" sound. Maybe you don't even want a modal or extended chord. You just want something weird and "out", or just spooky and spacey. That's when diminished, augmented, and altered seventh chords really start to shine. These scales will help you solo over these weird chords. These kinds of scales are used a lot in jazz, but creative guitarists like Satriani and Yngwie also use these kinds of sounds for variety - more than you think!
This is a symmetrical scale. That means it is based on a pattern of steps that doesn't change. For instance, a major scale pattern is 2 2 1 2 2 2 1. That's not symmetrical. But this scale is 2 1 2 1 2 1 2 1. That will get you back to where you started. There is no key implied, making it sound "homeless", unresolved, and "out". Because of its symmetrical nature, the scale is the same thing every three frets! That means there's only three possible diminished scales. This is one way to play it. I usually just start on the D string, it makes for a really easy scale to play!
This scale can be used for blues (use scale starting on Bb for C blues). It also works well over diminished chords or dominant sevenths with a flat ninth. Here's a lesson by Larry Carlton about this scale:
A "whole" tone just means two notes separated by two frets. In the music theory world this is also called a major second interval. This scale is also symmetrical and has the pattern of 2 2 2 2 2 2. This is also a "rootless" scale so there is no key. It repeats every two frets, so there are only two whole tone scales.
This scale is useful over augmented triads and dominant sevenths with a sharp fifth. Common in jazz, fusion, and blues. Here's a long video lesson on this scale and its uses from Andrew Wasson:
Also, here's a Satriani tune called "Trundumbalind" from the album Crystal Planet. The section near the end of it is entirely whole tone! Knock yourself out soloing along with him, it's actually pretty fun and sounds neat.
Joe Satriani - Trundumbalind
There are lots of other scales out there that people use to make sense out of soloing. I may come back and add some later just for fun. However, most of the soloing I do is in a key, so I'm always trying to get better at that.