The top and bottom reinforcement rings/hoops (inside) some drums.

John

Reinforcement rings -They are part of the construction of the famous 3-ply shells -which were very thin and lightweight on their own. So, solid maple reinforcement rings were glued to the interior ends of the shells to provide.....um....structural reinforcement! (Makes sense, no?). It's how to tell a 3-ply shell at a glance. If you see the re-rings then they are 3-ply shells (some exceptions, but as general rule this applies)

Hope this helps