We had an X aerial from about 1956 to 1960, because we had a television which could only be tuned to a single channel (BBC). My grandparents bought a new television in the late 1950s when ITV arrived, this had a rotating dial so you could switch between the channels. They added an H aerial to their chimney. I think when they switched between channels they had to unplug one aerial and plug the other one in. Both these channels were 405 lines. When BBC2 came along it was on 625 lines, so all change again.