Penthode has pretty accurately summarised the British era of low definition TV transmission. There were also similar experiments in other countries.
Don (not Doug) McLean has also written an excellent book. Restoring Baird's Image". Although out of print, copies are still available at fair prices:
http://www.amazon.co.uk/Restoring-Hi.../dp/0852967950
If you don't already have a copy then buy now before it becomes rare and expensive. If you have even the slightest interest in this era then you will enjoy reading this book.