Straight razor sizing query

Bristol, UK
My apologies if this has been answered before but I have searched but cannot find the answer.

I understand that generally speaking a popular straight razor size is 5/8's. (whether this means five eighths of an inch or not or not I don't know). Either way, one would assume that 8/8's was the maximum.

I have recently seen a number of straight edge razors for sale that are sized at 10/8 (ten eighths?). I'm no mathematician and nor am I particularly good at maths in general but I was wondering how that can exist as a fraction or a size?
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