Satch, Vai, Beck, Randy Rhoads, Hammett, etc etc are all examples of players who studied their theory (often classical guitar as well) and apply that knowledge to writing their solos.

The simplest way to look at it is to say "okay, the key of this song is A minor, so I’ll play the solo with the notes from the A minor scale".

Theory isn’t some "set in stone" process…. theory tells you what notes will probably sound best, ie which notes will give a certain "color" or "texture" when you play melodies based on them.

It took a long time for me to pick up on the concept of lead guitar…. but it all came from learning my scales, then just playing with them for hours and hours – not just learning the notes, but practicing patterns and fills until I became very comfortable with the ins-and-outs.

One way to look at lead guitar is that it’s a device for communicating a melody… the function of a solo, really, is to make a musical statement, and the most powerful way to do that is through an effective melody…. a melody meaning something you can sing along to! Think about it… how many solos can you sing along to? Are they not some of the best solos you’ve ever heard?

So one way to approach writing a solo is like a vocal melody… sing along to the chord progression and see what you come up with. Use that as your inspiration for your solo.

The major and minor pentatonic scales are the easiest to work with… they don’t have a strong "statement" to them, but they work very well at creating a framework to start with.

To anyone who says "just use trial and error"…. you’re absolutely right. You can do it that way. There’s 12 notes you can pull from, so knock yourself out and guess away. However, music theory will help you narrow that field down to 7 notes or less… in other words, you can cross out almost half of the notes right off the bat. Gee, does cutting your guesswork in half sound like a good trade-off?

Saul