Hi folks. Probably a silly question but what does NB in the blue rectangle mean? I can't see it mentioned on the Key/Legend.

Thanks in advance.

Can you give us a locale or a link to the route? Depending on location it it possibly a label for a cycling route. The National Byway perhaps if you're in the UK. (Found by googling NB & cycle route :-) )

It is the National Byway!

The little rectangles are route numbers or abbreviations. When following these routes you should see signs that (roughly) approximate to them. The National Byway signs are brown with a Hovis-style loaf logo for reasons lost in the mists of history.

Thank you Richard and Hobbes. Just to confirm, here's an example from near Bedford.