I've been watching a few 'factory tours' videos on youtube recently (PRS, Gibson and the like).
 
I know very little about woodworking and far less about making guitars.
 
My question is this: why do they take the piece of wood which is to form the body of the (electric) guitar, chop it in half then glue it back together again to make the body, rather than not chopping it in half in the first place?

A piece of wood that has been cut and glued is stronger and much less resistant to warping.
Also, on some guitar body styles it makes the routing for electronics much easier when the body is in 2 pieces. The routing gets done after splitting, then is glued together.

Michael (Spacey) would've been all over this.......miss him.

Randy P
A piece of wood that has been cut and glued is stronger and much less resistant to warping.
Also, on some guitar body styles it makes the routing for electronics much easier when the body is in 2 pieces. The routing gets done after splitting, then is glued together.

Ta :-)  I suppose it also allows for one half to be flipped, thereby creating the 'butterfly ink blot' type of pattern with the wood's grain.

ston

Randy P
A piece of wood that has been cut and glued is stronger and much less resistant to warping.
Also, on some guitar body styles it makes the routing for electronics much easier when the body is in 2 pieces. The routing gets done after splitting, then is glued together.

Ta :-)  I suppose it also allows for one half to be flipped, thereby creating the 'butterfly ink blot' type of pattern with the wood's grain.

Randy nailed it.  This process is common on table tops, cutting boards, etc...keeps its flatness.  
 
Ston, this is called "Bookmatched" on the maple pattern.  

Though bookmatching is attractive, the real reason is it saves money.
 
It is not glued back where it is cut. What happens is a thick piece of wood is cut in half and the resulting two thinner halves are glued together so that you end up with a piece that's twice as wide but half as thick. 
 
Cheap guitars with solid finishes (or thin veneers on top and sometimes bottom) often will have more than two pieces.
 
Single piece bodies and non-bookmatched tops are available, but they are generally more expensive.

The size and "consistency" of trees has a lot to do with it.
 
Once the timber's been sawn at the timber yard it's much easier to find quarter-sawn (cut at right angles to the grain) knot and void-free sections with a good consistent grain density 4 inches thick and 8 inches wide than 2 inches thick and 14 inches wide. So you find, for example, a good-looking 4x7 block and after cutting (or splitting) and glueing the thin edges together it becomes a 2x14 body blank ready for shaping. It's even harder to find timber suitable for one piece tops on acoustic guitars, with their typically wider body.
 
I've heard Fender (and many far-east mass produces) might sometimes use more than just two pieces - I've seen internet claims that a Fender body "spread" might contain up to five different sections. Don't know if it's true or just "internet experts", but if it is it kind of undermines the "one piece bodies sound best" argument. Once the wood's glued it's all a single resonant plank in any case, the glue's far stronger than the wood and it's rigid so it passes vibrations no problem. Too lively body resonance is a bad thing in electric guitars in any case, as anyone who's tried to play an ES330 or similar through a cranked amp will testify.

Interesting stuff, ta.  Also helps to explain why my one-solid-piece of mahogany chopping board which a friend made for me many years ago has a tendency to warp and try to split (I have to keep rotating it and keep it well oiled).
 
It is a damn fine board though, must have had it for over 20 years already.

I have a granite chopping board,  it doesn't warp at all but I wouldn't want to use it as a blank to make a guitar...

 

Karyn
I have a granite chopping board,  it doesn't warp at all but I wouldn't want to use it as a blank to make a guitar...

Why not? It would rock!