Dansm's Guitar Chord Theory
Alternate Bass Notes of Chords
The Infamous "Slash" Chords
Slash chords like G/F# seem to be the most well-kept secret of the guitar industry. No one wants to tell beginners what the heck that slash means. Well, I am going to go out on a limb and explain to you guys what the slash refers to and why you use chords with slashes. So stop being confused, and read on.

One note before we begin: I will be using many musical examples, so when I mention a chord progression, play it to see how it sounds so you understand what I am talking about. Good luck!
If you don't know the notes of the fretboard up to the fifth fret, you will need to know them to understand and to create your own slash chords. These are the notes of the fretboard:

Thanks to Music101.com for this image.

As I mentioned in the overview of keys, each chord contains a root, third, and fifth. According to music theory, the lowest note played (bass note) in any standard chord must be its root. Therefore, a D chord must have a D bass note. Therefore we get xx0232, which has a D bass (the open fourth string). However, music would get rather boring if every chord contained the root note as the bass note. Therefore, musicians frequently use alternate bass notes. If you want to change the bass note of a chord, you use a slash. For example, if you want a D chord with an F# bass, you write D/F# and play 200232 (thumb 6th string) or 20032x, because sixth string 2nd fret is an F#. Note that the D/F# chord contains all the notes of a D chord and an F# bass, not the other way around.

You may ask "why would you want to use an alternate bass note?" Say you are executing this progression: Em-D-G. If you throw in a D/F# instead of the D, the F# bass stands out clearly and leads the D chord strongly to the G chord. So you change your progression to Em-D/F#-G, and it goes together much better. Try playing both and see. However, this will not necessarily work all the time depending on the sound you desire and fingerings you are using so look at each indivisual case to see what's best.
There are many uses for these slash chords. The most common use is for a bass run. This is a rapidly changing bass line that drives your chord progression. This is outlined here: C-C/B-Am. C/B is played x2x010, and provides a good intermediate chord between C and Am. Another common bass run is G-G/F#-Em, which is basically the same idea as C to Am. G/F# is played 2x0003. For a song with extensive use of bass runs, see the Eagles' The Girl From Yesterday.

The other common use of slashes is to change the inversion of chords, which I illustrated above. When you have a D chord, the three pitches are D, F#, and A. If you play a D/F# (200232) or D/A (x00232), you are playing an inversion. Inversions use a chord tone as the bass note, while bass runs generally do not. When you consider C/B, B is not in the C chord (C, E, G), and therefore is not an inversion. But F# is in a D chord, so D/F# is an inversion. Inversions are used to bring out bass notes or to provide continuity between chords. For example, in the Eagles' Life in the Fast Lane, there is a progression in the middle which goes like this: E--D/E--C/E--A/E, all to provide the continuity of the E bass because the riff is based around that E. Obviously as I said before, E has an E bass. D/E is not an inversion, it is simply a continuation element. C/E and A/E are inversions, though, so it is not entirely strange to see them in that form. Here are some common slash chords that you may come across in everday music:

dansm rules x2x010

You may also want to eventually try to figure out how to make a weird slash chord. I will consider two types of slash chords to figure out. The first is an inversion, Em/B. A normal Em chord contains the pitches E, B, G, and is formed 022000. Notice that B is the fifth of the chord, so Em/B is an inversion, as I said. Since B is already in the chord, all you have to do is look for where that note appears in the chord. In our case, the fifth string 2nd fret is a B. If we treat that as the bass note, we get x22000, which still contains the pitches E, B, G. That means that we still have an Em chord, but with a B bass: Em/B.

The second type of slash chord you will want to create is an alternate bass that isn't an inversion. Let's consider an interesting chord named G/A. A normal G chord contains the pitches G, B, D, and is formed 320003. Notice that A does not appear in the chord, so G/A is not an inversion. What we need now is a complete G chord (G, B, D) with an A as the bass note. If you look for the lowest A on the fretboard, it appears as the open 5th string. Therefore, we will treat that as our bass note. Now, we must place the three pitches of the G chord (G, B, D) on top of that A. A quick glance at the normal G chord will give you an idea: simply use the xx0003. Therefore, combining that form with the open fifth string A, we get x00003, or G/A.
Now wasn't that easy? I hope this was a big help to people who are completely confused about slash chords. Good luck and have fun.
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