This lesson appears lengthy, but that's because there are lot of explanatory diagrams. The ideas are simple. If you read to the end, I'm confident you will have a good idea of how to make sense of the guitar fretboard.
I'm going to start on piano briefly, to get us going.
Look at this piano picture. Visually this has landmarks of a grouping of two black keys and another grouping of three black keys, repeating. Without these, players would get lost finding a particular pitch (each key on the piano produces a unique pitch... More in a moment).
Imagine playing the left key of the leftmost grouping of two black keys on the piano (an arbitrary choice). Move right one key (which will be white) and that key is a semitone higher in pitch. Keep moving right until you encounter the left key of the next leftmost grouping of two black keys. Where you are now is 12 keys (count for yourself) to the right of where you started... Its pitch is 12 semitones (an octave) higher than where you started.
You can keep doing this (another 12 keys to the right) until you run out of keys. Depending on the piano, this gives you somewhere from 5 to 8 unique pitches that are some number of octaves apart from each other. Sound frequency-wise, they may be something like 100 Hz, 200 Hz, 400 Hz, 800 Hz, 1600 Hz etc. (keeps on doubling... As that is the true definition of "octave").
Sound flavor-wise, they all sound extremely strongly related to each other. Pitch name-wise, they are all given the same basic name, but with a number tacked on to indicate exactly which one we mean.
In what we've just looked at, the basic name is C# (C sharp), so we have C#1, C#2, C#3... to name these. Using the same principle, if we started one key further to the right than where we started above, now you get D1, D2, D3, D4... and so on. Ultimately you get 12 such groups, each with a different basic pitch name, so we can talk about "all the C's" or "all the F's" and so on.
On guitar the same 12 groups exist, but there is more than one location (string and fret) that can produce the exact same pitch. Knowing the set of "octave shapes" solves this (refer back to the interval lesson).
Let's now swap to discussing guitar. Here's some terms for later:
"Moving up" (along) a string is towards the guitar body. It results in higher pitches, e.g. from open string to fret 1 is up. "Moving back " along a string is towards the nut. Moving up a string is like moving right on a piano: The pitch increases a semitone per fret. Moving back along a string is like moving left on a piano: The pitch drops a semitone per fret moved.
Where the two pitches of some interval are located on two different strings, the "upper" string is the skinnier string.
For example, in this next diagram, the A string is the upper string, relative to the where the solid red pitch is located. In standard tuning, the 5th fret of the bass string produces the exact same pitch as the open string (fret 0, the nut) on the adjacent upper string (the 5th string). The pitch is A2. (Hence why we call the 5th string the "A" string, or "open A")
Move up 5 frets on the 5th string, and that fret produces a pitch 5 semitones higher. The next diagram shows this with pitch names.
The next diagram shows the same thing, but now the upper pitch (labeled 5) is shown as being 5 semitones above the lower pitch (labeled zero). The red pitch is where we're measuring from to this upper pitch.
We've just seen that the 5th fret on the 5th string produces a pitch 5 semitones higher than the 5th fret of the bass string. If you fretted both these pitches (the A2 and the D3), the visual shape is a vertical line (vertical across the neck, parallel to the fret).
Slide that vertical shape anywhere along that string pair and the pitches involved obviously change each time, BUT... whatever those pitches are, they are always 5 semitones apart. Here are a few examples, labeled with pitch names, or with semitones. Which is easier to remember?
This interval involving two pitches 5 semitones apart produces a sound flavor unique to that interval size. As it slides the flavor is unchanged, but you hear this flavor higher or lower.
The above shape holds for every adjacent string pair apart from the pair (G, B), where the strings are tuned 4 semitones apart. In this case the vertical line shape produces an interval of 4 semitones. In this next diagram, everything is measured from the red pitch labelled zero. It's all still consistent... Look at the rightmost pair of pitches labeled 4 and 8. These are 8-4 = 4 semitones apart. You can see the 5 semitone shape (0 -> 5) at the left of the diagram. This had to adjust up one fret (compared to what we've looked at above) because the tuning means the straight line is 4 semitones.
Next diagram, all the above has been slid up 4 frets. Our measuring point is still the red circle (I'm using software to do this dragging about).
Adjusting the "Vertical Shape"If the vertical line, 0 to 5, is 5 semitones: then simply move up along the upper string 2 frets (2 semitones) to get 7 semitones. Slide the shape formed by fretting the 0 and the 7 along its string pair, and you'll always get 7 semitones between the two pitches involved
Here are a few examples.
Here is the same idea, but on the string pair (G, B) that are tuned 4 semitones apart. Again, slide the shape formed by fretting the 0 and the 7 along its string pair, and you'll always get 7 semitones between the two pitches involved. Any other string pair, use the above.
In general, here's how to find various semitone distances moving along up the string from the vertical line...
Each of the pitches on the upper string is labeled by semitone distance from the red pitch (our datum point).
The interval of 7 semitones crops up a great deal. E.g. next are a few major triads I'm sure you'll recognize. See if you agree with the semitone distances shown to the other pitches, as measured from the red pitch. It's important you try this for yourself... Just remember the number of semitones involved in the vertical line shape for a given string pair.
Here's the above chord slid...
Adjusting Down From a Vertical Line ShapeKeep moving the upper string pitch lower (towards the nut). The various semitone distances from the red pitch are shown:
This gives a very simple way to find the exact same pitch on adjacent string pairs. From whichever fret you are choose on the lower (bassier) string, cross strings to the upper adjacent string (the vertical line) and move back (towards nut) 5 frets. Conversely, starting on the more treble string, cross to the bassier string (vertical line) and then move up along that string by 5 frets. This works everywhere apart from the (G, B) string pair, where you use 4 frets instead.
About Chord Shapes and IntervalsNow, if I change the chord shape somehow, this completely changes the sound flavour. For example:
The change on the 3rd string drastically changes the sound.
If I want to create a major chord sound higher up the neck, then one way to do this is slide the shape unchanged up the neck. This means introducing a barre with your 1st finger, to play the role that the nut was doing for you (when it acted as fret 0). In this way, all the measurements we did above will be identical in terms of semitones from the pitch on the bass string. But the actual pitches involved will all change.
Musical Naming Convention for IntervalsWhile the above is absolutely correct in terms of measuring semitones from sound chosen pitch (always the solid red pitch in all the above examples), no one uses this numbering scheme. Instead, there are some music terms used. Here are a few:
"1" (unison) is the synonym for an interval that is zero semitones above the measuring point (i.e.coincident, i.e. is the measuring point), and
"3" (major third) is the synonym for 4 semitones and
"b3" (flat 3 or minor third) is the synonym for 3 semitones.
"5" (perfect fifth) is the synonym for 7 semitones.
In fact, these are used to represent all octaves of a particular distance. So if we have a pitch 3 semitones above our red pitch, then the octaves of that pitch are at 3+12, 3+24, 3+36... Likewise, octaves of our red pitch are at 0+12, 0+24, 0+36... Octaves of the pitch 7 semitones above the red pitch are found at 7+12, 7+24, 7+36... You get the idea. Using the correct naming in the above two chord diagrams, we get:
So, for example, we can look at the left diagram, and talk about the major third (labeled with its musical name "3" for the interval), or look at the right diagram, and talk about its minor third (labeled "b3"). This is what distinguishes a major from a minor triad, and causes the huge change in sound. This b3 or 3 is an important pitch to target when soloing or singing over a minor or major triad.
And here are the same two chords, slid up three frets:
This is why thinking of shapes as involving a set of intervals measured from one of the pitches in the shape (which we call the root, for a chord, and tonic for a scale) is so much easier. I don't care what the pitch names are, and never think of them (other than where the pitch for the root or the tonic is located), because it is the intervals involved that's responsible for the sound produced. Imagine looking at all the above, and discussing it all purely using pitch names. Way, way harder, and much less useful.
When we write a tune, we choose a bunch of intervals to work with (for example the "major scale" is a shorthand way of talking about the set of intervals found at 0, 2, 4, 5, 7, 9 and 11 semitones from our choice of tonic). The choice for tonic is normally based on how comfortable the resulting pitches are to sing or play. Let's choose E2 (fret 0, 6th string). (see diagram below). If we choose G2 (3rd fret, 6th string), we get pitches at fret 3, 5, 7, 8... As these are 0, 2, 4, 5 semitones above G2. You starting to get the idea? And remembering that are several octaves of any pitch, we use them as well (not shown below).
Here it is, the E major scale, labeled counting semitones from the E (red pitch):
and here's G major scale, again labeled counting from the red pitch:
Here's the G minor scale (semitone intervals of 0, 2, 3, 5, 7, 8, 10). Using musical terms this becomes (1, 2, b3, 4, 5, b6, b7):
and labeled musically:
The thing to realize about a scale or chord (as we've just seen) is that these are formed by intervals from the tonic (scale) or root (chord).
So, if you know an interval shape (where you place your fingers... So many strings and frets apart), then you can see it in both chords and scales. For example, here's G minor pentatonic (1, b3, 4, 5, b7). Compare with above. If you encounter any scale or chord that includes a b3 (3 semitones) or a perfect 5th (7 semitones), you know how to find it. When you solo, you know where the important intervals are in the chord or scale.
They help you remember how to form the scale or chord. They are the absolute foundation stone of all music.
By knowing the basic interval shapes (as in the original interval lesson) you reduce your mental effort massively.
Finally, music ultimately comes down to light and shade, tension and resolution, and phrasing... This is all controlled through interval usage, and rhythm.
I hope this helps clarify for you, and gets you going on your musical journey.
My guitar playing can be heard here.