From International Owl Center

Owl bills and talons are essentially keratin sheaths over bone. When we trim their bills and talons (coping), we have to be sure not to trim too close to the bone, just like you don't want to trim dog and cat claws too short or they will bleed.

Some species have light colored bills, like American Barn Owls and Eastern Screech-Owls (like JR, pictured here.) With some backlighting you can see the bone inside the bill, which makes it far easier to know how much we can safely trim.

This isn't the best photo since JR doesn't sit still well, but if you squint just right you can see the bone inside his upper mandible.