I am sure many of you newcomers to the instrument have a common reaction when you learn about tuning: "What the heck is with the B string?". Whenever I teach someone how to tune a guitar for the first time, they always ask "why is the B string tuned at the 4th fret instead of the fifth like the others?". I always answer pretty much the same way:
"Because it Just is"
(nice teacher huh?) I usually follow that with "You will grow to understand why as you learn more chord forms". This really is the crux of the reason (I mean aside from the uneven tone/semitone pattern the musical notes are governed). It just ISN'T mathematically even. But rest assured, knowing that the designers of the instrument were pretty smart cookies. Imagine all the chords you know, with the notes on the high E and B string back one fret - what a mess! This offset truly makes the best of a bad seeming situation. So let's just accept it as it "is" and discuss understanding how to deal with it.
The Conveyor Belt
Many different people over the years have come up with many different ways to describe the tuning anomalies affect on fingerings. My favourite is Jon Finn's "Warp Refraction Threshold". To me, it is most simply described as being akin to a conveyor belt; meaning all notes on the B string slide forward by one fret in order to correct the offset in tuning. Like on an airport conveyor belt... any note on it makes the journey forward before continuing on.
Click on the images below and you will see animated versions that demonstrate. As you can see, if the guitar were tuned straight across at the 5th fret, then the high two strings would become C & F (as opposed to B & E). I created a uniform geometric scale fingering to more clearly show how the shape is effected by the 4th fret tuning. As you cross from the G to the B string, there is a momentary shift upwards by one fret. So, to reiterate, the effect happens as you cross from the G to B string.
With this in mind, let's look at how this offset affects chord forms. In the below diagram (again, click to view), I placed a Major barre chord form. Let's say we want to move that whole shape up to a higher string. As you cross the B string you see that we need to raise the note on that string (conveyor) up by one fret. The resulting shape should be familiar to most of you.
This anomaly effects all shapes on the guitar. Chords, scales, you name it. When you cross the B string, you have to alter your pattern to adjust. One of the strengths of the guitar is that it is very shape-friendly... meaning you can take any pattern, starting on any note, and apply a shape to it. This results in the desired chord or scale. The problem is many newbies fail to make this observation; giving them the unnecessary feeling that there are a vast number of more shapes to learn.
So take a minute and spend some time drawing out some scale forms and chords. I think if you look closely, armed with this post's info, you will discover some things that may have eluded you in the past.