Neck Joint - Neck Through

Neck-through or neck-thru (or in full form neck through body) is a method of electric guitar or bass guitar construction that involves extending the piece (or pieces, in a laminate construction) of wood used for the neck through the entire length of the body, essentially making it the core of the body. The strings, fretboard, pickups and bridge are all mounted on this piece. So-called "ears" or "wings" (i.e. side parts of the body) are glued or laminated to the central "stick". The "wings" may be bookmatched in order to give a symmetrical appearance, and are often cut from one piece of wood.

Neck-through construction is significantly harder to mass-produce than bolt-on or set-in neck constructions and is primarily found on high-end guitars. It is somewhat more common in basses than in guitars. Neck-thru construction allows easier access to upper frets, because there is no need for a heel — the thickened area where a neck would "bolt-on" (although technically, 3-4 large screws are used) to the body. Many musicians assert that neck-through construction provides greater sustain and stay in tune longer. There is little formal research on this topic, but what exists does not support this assertion.

Repairs to the neck are usually expensive and tedious. In many cases, it is usually easier to remove the old neck completely, either by taking the wings off and putting an entirely new core in, or by converting the guitar to a bolt-on or set neck by creating a heel and affixing the new neck to the core already in place, rather than to try and repair the neck itself. However, thanks to excellent stability and reaction to string tension and pressure, neck through guitars are often much more sturdy than many other guitars on the market.

source : Wikipedia