What order are couplings like 'CharAxCharB' in? In doujins coupling characters from a show off, the characters are usually referred to in the title as "Character A x Character B"

(I feel this snippet from hanagai! explains it better)



Is there any meaning to the order of the 'multiplying'? 

Is the first character mentioned the main character, more dominant, alphabetically first, randomly chosen or something else?
The first one is the dominant one a.k.a. seme while the second one is the receiver a.k.a. uke. For example in doujinshi Puella Magi Madoka Magica Doujinshi Homura x Madoka (available online in online manga sites), Homura will be the seme, while Madoka is the uke.