Interpreting Engineering Drawings

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How to read screw thread size?

I am currently doing an engineering assignment where i have to model a non-return valve using computer software. On the drawings given there are screw thread dimensions given as follows.
m42x3-g
m42x3-6h
etc.
I know that the m42 means the outer diameter is 42mm but I am unsure of the rest. I assume the 3-6 specifies thread pitch but am unsure how to interpret and what do the letters mean?
Thanks for any help, Elliott

Hi Elliot, maybe wikipedia can help?

http://en.wikipedia.org/wiki/Screw_thread

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