I have seen the wording "long diagonal" in several chess books.  I had to use almost all of my fingers, but I finally was able to count the squares -- eight!  Just to check, I counted the squares in the other diagonal too, which, by way of comparison, surely must be the short diagonal.  And what do you know?  Also eight!  Is this some kind of new math?  Or similar to when someone breaks the candy in half and then takes the larger half for himself?!?  Hopefully someone out there can see the source of my confusion and set me srtaight.  Thank you.

Those two are the longest of all the diagonals.

So you mean the comparison is to the myriad of shorter diagonals, which do not extend corner to corner?  I guess that does make sense.  Thank you!

a1-h8 is the longest diagonal the black bishop can use, and a8-h1 the longest for the white.

Because of collor that is mutually exclusive to each other.

One reason the books talk about long diagonals is because they're where a bishop is most powerful.  A bishop at the center of the board (say on d4) has access to the 2 longest diagonals available to it and can potentially influence 13 other squares.  A bishop at the edge of the board (say on c1) only has access to 2 shorter diagonals and can only influence 7 other squares.

Thank you for clarifying!

I think the reason that they say "The long diagnol" as if there is only one is because their is only one "long diagnol" per bishop.  So there is only one diagnal which applies to the bishop in question.  Did that make sense?

Yes, sort of a stratified comparison within colors.  Thank you!

I think most of the time when books use that phrase, it's just a figure of speech.  It usually just means that the bishop is exerting its influence on a sector far from where it sits; it doesn't have to be the longest diagonal on the board.

Good point -- thank you!