A bicycle rim has a sidewall on both sides which acts to hold the bead of a tire in place so it's doesn't blow off the rim when inflated. On the inside edge of the rim sidewalls it's common for there to be a small hook which helps to hold the bead of the tire in place but some newer rims are opting to go hookless. Hookless rims allow tubeless tires to setup better since there is a uniform surface for the bead to contact which makes it slightly more air tight. It's still possible to setup a tubeless tire on a rim with hooks but it's won't hold air as well. At the base of the sidewalls is the bead seating ridge where the bead of the tire sits. Between the sidewalls is referred to as the well of the rim since it dips down. The rim well is where you'll typically find the spoke holes that have been drilled into the rim. If there are no spoke holes on the inside of the rim then they may be covered by rim tape or the rim could be a UST rim. UST rims are specifically designated as tubeless since there are no holes drilled in the rim for air to escape from. The nipples that spokes attach to are generally threaded into the rim.
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