So I recently re-strung my very old and very beat acoustic guitar, thinking I'd start playing a little. Well...I don't remember jack. And worse, the guitar lessons I took as a kid and teenager taught me VERY little about music theory and a lot about playing by ear. I don't remember anything useful of either. But I'd like to go through some music theory lessons to remember how stuff works...scales, progressions, mathematics of music, etc. Are there any good places to learn online for free?