What made King George V-class belt armour distinctive?

What was special about King George V-class slab armour that could not have been reproduced earlier?

Its thickness and material quality were the main advantages. An earlier arrangement was possible, but would not have offered the same quality of protection.

The advantage lay in the armour's thickness and particular material quality. British and American armour are placed at the centre of the comparison, with the ranking dependent on the sources and tests used. Vanguard, Anson and Howe receive particular praise for their plate quality. An unpublished paper is mentioned in connection with the possibility that a King George V-class main belt could stop a first hit from a Yamato 18-inch shell. That remains a debated assessment, and the accompanying claim is that a second hit would penetrate. It does not establish general immunity to Yamato's guns.