“Mind the gap” is a warning to tube passengers to remind them of the sometimes significant gap between the train door and the station platforms. It was first introduced in 1969 by the London Underground system.
Slate traditionally has quite a significant gap between tiles for the grout. Slate tiles are not usually sized as precisely as some other types of stone tile, and because of this difference, a wider grout joint helps with the alignment process.
Also, since the top surface of natural-face slate tiles are slightly uneven, a wider grout joint is used to help smooth the transition from one tile to the next.
So how wide should the grout joints be? Well, It’s a matter of personal taste, but the norm is around 3/8″ for sizes up to 12″x12″ and 1/2″ thereafter.
And what type of grout should be used? – Use a sanded grout for joints of 1/8″ to 1/2″ and a saltillo grout for joint widths of 1/2″ to 1 1/4″.