So-called "K-shell" x-rays are produced when electrons fall from outer electron orbitals to the innermost. (The "L-shell" x-rays are produced when a vacancy is filled in the second-innermost shell.)
An electron falling to the hydrogen ground state emits ultraviolet radiation in a spectrum first observed by Lyman. That is, the physics of the hydrogen Lyman series is mostly the same as the physics of K-shell x-rays in heavy elements. Likewise the physics of the Balmer series of visible-light hydrogen transitions is analogous to L-shell x-rays in heavy nuclei.