Here's a handy table that helps you know which metric wrench (or spanner) to use with a given thread size bolt, along with the typical pitch. Or it can be used to figure out the thread diameter based on the nut size or hex cap size.  I keep a copy in my toolbox.

It's easy to get confused when you use a 10mm wrench to remove a bolt and then someone tells you you need an M6 x 1.00 pitch bolt of x length. Huh....shouldn't I get a 10MM bolt? Or you'll see that the Clymers will give you a torque setting for a nut that fastens to a 9mm thread and you have to use a 14mm spanner.  Huh?

Note also that it is a good thing to have your own metric pitch gauge so you can determine if it is a std, fine or japanese pitch.  Usually it is just std or fine.

Hopefully, this will clear up the mystery. And didn't they tell us in school that the metric system was easy?