What does the “3,4” mean in 3,4-methylenedioxymethamphetamine (MDMA)?

I'm curious to know, what exactly does the 3,4 part of 3,4-methylenedioxymethamphetamine represent?

• Actually, this notation is pretty common, and not just for aromatic rings. Basically any time two isomers can exist that would have the same basic systematic name (for instance, dimethylpentane), and it becomes necessary to specifically identify the permutation, you start numbering the carbons in the chain or ring and identifying to which of these carbons other functional groups are attached (dimethylpentane for instance can have 2,2, 2,3, 2,4, and 3,3 isomers; attaching a methyl anywhere else either makes the pentane into hexane, the methyl into propyl, or is a "synonym" like 4,2 or 3,4) – KeithS Sep 16 '13 at 19:29

Numbers in organic molecules tell you where so-called functional groups (important configurations of elements) are attached to a central structure. The central structure in MDMA is a benzene ring $\ce{(C6H6)}$, with the main functional group attached at a carbon which makes methamphetamine. This carbon is labeled 1 as the "most important" functional group. To get to MDMA from methamphetamine, you add a methelynedioxyl group, and this attaches at carbons 3 and 4. So that's where those numbers come from!