The X-rays emitted seemed to be of two types. One type was able to penetrate sheets of metal of a certain thickness; the other wasn't. The spectroscopist who discovered this (Charles G. Barkla) named the more penetrating type A and the less penetrating type B, initially. But he worried that even more penetrating types of X-ray radiation would be discovered for other elements, so he renamed them K and L to leave room for more penetrating types A through J. These more penetrating radiations were never observed.