Did you know that if you learn four different (but similar) “Barre chords” you can play along with thousands of songs? It's true! By learning just one barre chord, you could play at least a thousand songs without having to remember where each individual finger needs to be placed for each chord.
When you pluck a guitar string without placing any fingers on any frets (known as an "open" string) it vibrates between the bridge and the nut at a specific rate. In the case of an acoustic guitar the vibrations of the string create a sound frequency audible to human ears. When you strike a string on an electric guitar the vibrations of the string create a changing magnetic field, which is converted to a changing electrical signal by the guitar's pick-ups, and is then sent to your amplifier.
If you make a string shorter, the vibrations will be faster making the pitch of the note higher. When you place a finger on the fretboard, you are in effect, making the string shorter, which results in a note of higher frequency and pitch.
A barre chord is simply a chord that is formed by using one of your fingers as a "capo". If you don't know what a capo is, let me explain: Guitarists use a capo - which is a clamp or vise of sorts - placed across all six strings in any fret to change the tuning of the guitar to a higher pitch or key. Without a capo you would need to manually tune and re-tune the strings when you want to use standard chords at a higher pitch. Using a capo allows you to play simple chord formations such as a standard C, D, G etc. and produce chords of higher pitch. Also, when you play standard chords with a capo attached it gives simple standard chords a brighter tone.
All of that makes a capo a cool tool, right? Yes, there are situations where a capo is very convenient, and times when using one is essential. However, you can emulate a capo by simply "barring" all of the strings with one or more fingers. Instead of clamping a device onto your guitar neck, you can use your index finger to serve as a dynamic capo, and form other chords all the way up and down the neck with just a few minor changes to the positions of your other fingers!
Let's look at four different barre chords that can be moved and used interchangeably to play hundreds, and even thousands of songs. We'll start with what I call the "E barre" formation.
PLEASE NOTE THAT, BECAUSE THE MAJORITY OF GUITARISTS ARE RIGHT-HANDED, ALL FIGURES AND IMAGES SHOWING BARRE CHORD FORMATIONS, FRETBOARD PATTERNS, AND SCALES ARE MIRROR IMAGES OF A RIGHT-HANDED PLAYER. THEY DEPICT WHAT A RIGHT-HANDED PLAYER WOULD SEE WHEN LOOKING AT THEIR FRETBOARD IN A MIRROR WHILE PLAYING. I APOLOGIZE TO LEFT-HANDED PLAYERS. I WILL TRY TO INCLUDE MIRROR IMAGES FOR THEM SOON.
If you don't know how to form a standard 'E' chord, look at Figure 4-1. Notice that the fingers are labeled thus: Index = 1, Middle = 2, Ring finger = 3, Pinky = 4, and the strings are labeled 1 to 6 (the thickest string being 1, and the thinnest being 6)
So, 1 is on the 4th string in the First fret, 2 is on the 2nd string in the Second fret, and 3 is on the 3rd string in the Second fret. Finger 4 is not used.
You can adjust or substitute the fingers being used in Figure 4-1 and create the exact same chord. Place 2 on the 4th string in the first fret, place 3 on the 2nd string in the second fret, and place 4 on the 3rd string in the second fret (See Figure 4-2). That is precisely the same chord formation, but using different fingers. Practice this different finger configuration by removing your fingers from the fretboard and putting them back on the strings in this new order and placement several times.
When you are able to place them there without having to think about it each time try this: After placing your fingers in this new 'E' formation, slide them halfway up the neck until your middle finger is in the 6th fret, and your ring finger and pinky are in the 7th fret. Next, stiffen your index finger and place it across ALL six strings. Figure 4-3 shows how the barre chord is formed in the 5th fret. Try to push them all against the fretboard. Don't allow your other fingers to move. Use the mid-section of your thumb to press against the back of the neck. It might take a while to become accustomed, but with practice, your hands will get stronger and your fingers and wrist will become more flexible. Play each string, one at a time, until each note in the chord is clean. You might need to make small adjustments to your finger positions to avoid muting some of the strings or causing them to buzz.