Do you Know why the keys on a keyboard are not alphabetically placed?

Way back when, before computers, even before electric typewriters, typing machines had the keys in various order. The letters were on long arms that hit the ribbon & paper when the keys were depressed. Unfortunately, these arms had a tendency to jam together if one typed too quickly. Motion studies were done, as well as letter usage frequency studies, and the QWERTY keyboard was designed as having the least tendency to jam. (Anyone who has used a manual typewriter knows the keys jam anyway, but this order was supposed to keep it to a minimum.)

I read somewhere that it was decided by frequent uses. I guess I could see it, as most of the letters in my screen name alone use a lot in the middle row.

As the others already said, the layout of a keyboard is designed for usability, based on how often certain letters are used. There's supposedly been keyboard layouts that were more efficient, but they turned out to be *too* efficient for old typewriters and caused them to jam.