Birch is part of the Yamaha heritage, and Absolute Birch has been the industry standard since it was introduced. With a new mounting system, the optional hook lug, and some hot new finishes, the bar has been raise once again.
First introduced on the flagship PHX series, the hook lug is a giant step forward in lug design. This lug anchors to a square, hooked post that anchors to the shell at the nodal point using a single bolt. This design reduces the number of holes and retains the integrity of the shell. The small contact area of the post allows it to make very little contact with the shell, and reduces the amount of shell vibration that is dampened. The hook lug also makes changing heads much quicker, since the tension rod only needs to be loosened a few turns, allowing the lug to detach the lug from the post.
 "Yamaha Drums - Yamaha Birch Custom Absolute Hook Lug Component Drums")