Understanding cartridges ?

So how do you determine if an arm is low/med/high mass?

"effective mass" is an arm manufacturer's spec. - just ask them.
Then read this link, & browse around all the other analog features on this site for a better understanding:
http://www.gcaudio.com/Archives/tonearmcartidge.htm

Thank you all for the great info. How important is arm/table matching? Suggestions for such, remembering small budget.
This was my post.

Thanks for the link, Bob. I'm glad I learned something about this before I purchase a second cartridge for my mmf-7. I have been able to find that the Project 9 arm is 11 grams and thus a medium mass arm. I have not been able to find the compliance rating for the shure v15vxmr that is currently mounted on the arm. Does anyone know what the compliance rating is for this cartridge?

I found it on vinyl asylum. The compliance is listed there as 25x10-6 (high). I thought that it would be a high compliant cartridge, but it is nice to see the numbers.