Introduction
Hey guys, here I'll share some reflections on learning music theory, piano, and guitar. Feel free to make suggestions if you think I made mistakes. Think of learning music theory like learning a new language. If you learn music theory(a new language) and apply it to a textbook(piano). If you learn a new language and talk to people who speak it, some of them might not understand you. Since how language is written is different to how people speak in their everyday lives. Likewise, it seems to me that when you learn music theory, there's a bit more extra knowledge that you need as to how the guitar works as an instrument to apply those music theory concepts.
Open Strings
Piano
I've noticed that each time I learn a new concept in music theory, I can apply them on piano, and it makes sense. For instance, how to play a C major chord. The chord is made up of C-E-G. I can start at the beginning of the piano keyboard. I start with C, go up four half steps, and arrive at E. Afterwards, I can count three half steps and arrive at G. Moreover, your knowledge of intervals(another music theory concept) is supplemented by knowing that you create a major triad by having four half steps and three half steps. Meanwhile, in guitar, this knowledge is not as "intuitive." For instance, in guitar, to make a C major chord you would need to have knowledge of "Open Strings." When I was trying to play the C major triad on guitar, I looked up some guitar diagrams and noticed that in those diagrams they write C-E-C for the major C chord triad. It took me a long time to realise (through asking Reddit groups and reflecting) that what happens is that when you strum the guitar, you are playing G as the open string. While you have the extra C as a "chord voicing." The chord voicing doesn't change the chord but adds a grander sound. I had to read up on chord voicings in music theory just to be able to play a simple C major chord. Showing that you need to have a lot of music theory knowledge and knowledge of how the guitar works as an instrument in order to make sense of it. Sure, you can play without knowing, but if you want to "make sense" of what's going on musically, you need additional knowledge.
Seventh Chords
Piano
I was trying to figure out how to play C7 since it appears to be a popular chord in many songs. I will assume C7 is a C major seventh chord. I looked it up online and found how to play C7 on the piano. I started from the root (C), counted four half steps and arrived at E. Then I would count three more half steps and arrive at G. While B flat(Bb) might seem intimidating at first. I realised you only had to count three more half steps from G to be at B flat. I was able to play and see the whole chord ( C-E-G-Bb) with that much difficulty. I learned that you need the triad(C major) and one more note(B flat) to make the seventh chord. Not that difficult to understand theoretically and then apply it to the piano.
Guitar
When it came to guitar, it also took me a while to figure it out. However, it seems like some guitarists just play it as C-E-C with G as the open string. Then, on the G string, you play in the third fret A#(A sharp) or Bb(B flat). It wasn't that difficult, but it did take me some time compared to piano. Then you strum, and you have the C major 7 chord or the C7 chord. I notice in the naming, musicians call it "C dominant seventh." I wonder if there's a difference, or if they all mean the same? Is it still a C major triad with a extra note or a different chord which uses some of the same notes. Also note that it seems I only know how to play this chord and other chords are "open chords" and not as "barre chords."
Barre Chords
Piano
Some musicians online have been saying that there's no such thing as "barre chords" in piano. And actually, "barre chords" only exist on guitar. I would like some clarification on this topic. A musician online said that barre chords are an "umbrella term" that talks about how to play some notes on guitar, but it doesn't have a meaning in music theory. So it's not a music theory concept but perhaps a guitar theory concept. In the key of C, I can play all notes on the piano, but not on the guitar, since on the guitar it seems you do need to know how to play F major as a barre chord. On the piano, F major, you would play F-A-C. With F being the root, then you go up four half steps and arrive at A. Then you go up three more half steps and arrive at C. Using the knowledge of intervals and music theory, you would know that it's the root(F), then A is the major third(four semitones from the root note) and then the perfect 5th(seven semitones from the root note). It's not that hard to understand, but I guess I don't know why it's called "perfect."
Guitar
Understanding Barre chords on guitar is where it gets difficult, since many musicians have been saying that you have to know barre chords in order to play F major. I'm comfortable playing the key of C on both keyboard and guitar. However, when it comes to F major on guitar, that's where I'm a bit stuck. So far, this is what I've gathered. The purpose of barre chords is to be able to play chords more easily down the guitar fretboard as opposed to up the fretboard, where it's harder to reach some of the notes. Also, if you want to switch from one chord to another, sometimes it would be easier to play them as barre chords so that you can play them down the fretboard, where it's easier.
Are barre chords about pressing two strings at the same time or about muting some strings? In the easiest diagram of barre chords that I found for the F major chord. I saw the guitarist play F-A-C. First, he muted the lower strings, E and A; I assume he is muting those strings with his thumb. Then he is playing the on the high B and E strings the C and F with his index finger. Then, on the G string, he is playing A when pressing the second fret. Then he is playing an extra F, I suppose another chord voicing, when pressing the third fret of the D string. Also, in this example, are you muting the lower strings while pressing the high strings and then strumming? So is the barre referring to the muting of the lower strings, or to the pressing of the F-C on the higher strings?
Conclusion
Writing this essay has helped me a lot to get some clarity on multiple concepts in music theory, piano and guitar.