Twist Doorbell Installation

I purchased a mechanical twist doorbell. My question is WHERE, on the door, were these traditionally mounted? Were they mounted on the SIDE of the door (above the doorknob)? Or, were they mounted in the center of the door (where door knockers are usually mounted)? Thank you.

Giorgina: I am so terribly sorry that I haven't been able to answer literally dozens and dozens of questions that readers have sent us. We're just so swamped and don't have the time to keep up anymore.

Rather than leave them waiting in the queue, I've decided to post the questions here anyway in hopes that other readers might be able to help answer some of these questions. Please, if you're reading this and want to help out, leave your comment and try to help if you can. Thank you!