Here is your basic dreadnought. 

 
 
 
 
 

14 frets clear of the body, we know these beasts well.

 
 
 
 
 
 
 
 
 
 

The distance from the nut to the 12th fret defines half its scale. 

Double that distance, and you have the full scale, the vibrating stringlength.

The length of that scale, not to mention the bridge location, is completely aribitrary.
 
If you shorten the neck, but keep the bridge in the same place, you have to compress the scale:

Look where the octave is now. It's as though you took the original guitar (at left) and removed the board between the nut and the second fret.
 
Same body, same everything, but progressively shorter neck

14 frets clear                            12 frets clear                              8 frets clear