Lonnie Donnegan and Bill Hailey both hit the Fifties somewhere around the middle (in Britain) and the Fifties can be divided into before them and after them.

Although musically there was a lot of overlap. Hank Williams was 'before' although he sang 'Move it on Over' in 1947 or 48 and Bill Hailey copied it around 1954.

And 'before' style songs like Wayward Wind and 'Out of Town' were being sung during the 'after' period.