I am taking 1/2 hour lessons in classical guitar and progressing well. I guess I am at the high end of beginner level, just finishing Aaron Shearer's first book. But I don't know how to gain an understanding of musical theory and don't know how to go about acquiring it. My teacher is starting me on R. Filiberto's Guitar Position Studies, but I look at page 1 and I don't understand the terminology. Any suggestions?
You are right. Guitar method books don't teach much traditional music theory. This is because the guitar is not an ideal instrument upon which to learn music theory.
Traditional Western music theory, as it is taught in colleges, is based around choral music. You learn to read and analyze choral music, and later to arrange and write your own chord progressions to be sung by soprano, alto, tenor and bass, in the "common-practice period" style. Put simply, this is studying the way that choral music composers from the time of J. S. Bach (roughly circa 1700) and later wrote church hymns. The reason for this is that if you learn this period and style of music, it encapsulates all the basics of Western music theory, and lays the foundation for all the music theory used in every style of music that came afterward, instrumental or vocal. Once you learn the fundamentals of choral music, you can go on to study areas of music theory that relate to all the other kinds of Western music.
Traditional music theory is taught by working examples out on sheet music paper and also working out all the intervals and chords by playing them on the piano keyboard. It's a given that music theory is much easier to visualize and learn on the piano than on other instruments, including the guitar. You don't need to become a piano player to achieve this; you just need to be able to find your way around the keyboard. Any four-octave MIDI controller or keyboard synth will do.
If you don't want to go into traditional choral music theory (which is a two-year college curriculum), you could suppliment your guitar studies by taking piano lessons with a teacher, while using a piano method book which incorporates music theory lessons from the piano perspective.
Find a piano teacher who understands that you already know how to read music on guitar, and that you want to learn music theory more than to learn how to become a pianist. It may be hard to find a teacher who is flexible enough to teach you what you want to learn, rather than insisting that you follow a beginning piano method book page-by-page.
There are method books for jazz guitar music theory, but jazz theory is an advanced form of music theory that is built upon basic traditional music theory. So you might find that going straight into jazz guitar music theory without knowing the basics might be confusing and not very productive.
Get a teacher.
Nice and simple. One with a college degree where he or she did at least four years of theory studies. Ask him or her what kind of melody and harmony work he or she did.
If he or she can write fugues and has a good knowledge on counter point then he or she would get you far.
Just remember that for every 10 practical teachers there may be one theory teacher worth his salt. The general feeling of theory among teachers is apathy. So getting one that sees the worth and also has proper training is hard. You are not talking about your average teachers here.
If you can find one though I can assure you that your musical endeavors will be greatly enriched with them.
You can go to a music shop that is well stocked with sheet music, method books, etc., and ask them to point you to the music theory workbooks section. Pick out Volume 1 of each series (all the major publishing houses have such a series), take them over to a comfortable chair, and choose one that appeals to you.
Please don't be put off by the small amount of material that is covered per volume in most of them. Just enjoy breezing through Book One and look forward to Book Two.
Each volume typically costs no more than $10.
Traditional music theory is basically diatonic harmony. I disagree with previous poster saying that the guitar is not an ideal instrument to learn theory on. Of course it's ideal, because it's a harmonic instrument (you can play chords on it). It and the piano are both ideal to learn theory on.
Plenty of music teachers can instruct you in theory. And there are plenty of jazz books that start with the basics. So pick one of those and have at it.
A lot of folks here are talking about theory as it's taught in the college curriculum, in other words, common practice era voice leading and analysis. If you're having trouble with Filiberto's book you probably need to work more on what is sometimes called "music fundamentals" rather than more advanced "theory" (although "theory" is often used in a way that covers both terms). These "fundamentals" would include minor and major scale formation, the function of each note in the scales, how the scales relate to key signatures, the triads (chords) that are built from each scale degree, and the usual diatonic function of each of those chords. This knowledge is often assumed by a college course in theory.
Personally, I learned some of this by reading online, some by playing trumpet in high school, some in learning (and especially improvising on) the piano. Taking college courses definitely led to mastery of the fundamentals, but I would never have survived them if I didn't already have a deep understanding of the concepts I listed above.
One book I can recommend that has a decent overview is Ralph Denyer's Guitar Handbook. It's more pop/rock focused and won't help you play classical, but it does go over music fundamentals pretty comprehensively and relates them to the fretboard. Unfortunately, it has a lot of other stuff that, as a classical player, you may not find interesting, but if you can find a cheap or loaner copy (your local library perhaps?), it's definitely worth checking out. The theory "method" books mentioned by other folks are also worth checking out, though, for my learning style, they were not very helpful.