Diagonal Dot Pitch?

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Hi, I'm looking for a monitor for the gaming PC I will be building soon. I was thinking of getting a 21'' or 22'' inch monitor and I came across the ViewSonic G220fb. It claims to have a .21mm dot pitch which I thought was very good considering the other monitors I looked at like the NEC MultiSync FE2111SB have a .24mm dot pitch. I noticed on the ViewSonic, however, that it also specifies a .25mm diagonal dot pitch. The NEC monitor didn't list a diagonal dot pitch so I am curious as to what the difference is. Which do you think would be the better monitor for playing games? Any input is greatly appreciated.