Journal Volume or Number?

Question

In the citation

Proc. Amer. Math. Soc. 2 (1951), 839-848

originally found here:

http://www.ams.org/journals/proc/1951-002-06/S0002-9939-1951-0045941-1/

How should I interpret the 2?

Answer 1

If you look at the part "1951-002-06" of the cited link or follow the link on the page to "this issue", you would have found out that this article is part of "volume 2, number 6".

Speculation: Since most libraries will have since combined the issues of that year into one book, i.e., you will rarely find the journals as separate entities, they have left out the number in the recommended citation format.