One of the features of my guitars is the carbon-graphite reinforcement inlayed inside the neck. I use a 10 x 3 mm bar, which gives the neck stability while remaining light and flexible, too. This also makes the neck-to-body area stronger, since it crosses it. Here is the bar ready to be glued into my new Stage Guitar (more on it later).
The bar is set into the neck using epoxy. Epoxy is needed because it bonds well to the carbon-graphite and also fills all the spaces in the trough. A maple veneer is laid over the bar to provide wood to wood contact for the fingerboard.
When the epoxy has cured, the maple is leveled and the fingerboard glued with hide glue. Hide glue has the best properties of any adhesive for wood-to-wood joints, resisting creep better than others. This is because of a molecular bond.
Utilizing a carbon-graphite reinforcement and hide glue results in a neck that will be both very stable for decades to come and light, flexible and resonant, which helps to produce the very best sound.