Button acordions/melodians are usualy reffered to by the number of rows of buttons and the keys the rows are in. So a two row B/C box tha two rows of buttons, the outside row (the row fartherest from the bellows)is in "C" and the inside row (closest to the bellows) is in "B". A lot of Irish players use two row B/C boxes some two row C/C#, most English players play G/D. Melodians are usualy single row boxes in one key, "G", "D", "C".Melodian style playing generaly means just playing on one row and one key, while accordian style means playing accross the rows picking out what flats and sharps are there to play is several keys. The Traditional Music and Song Assosiation of Scotland, The TMSA, has a rulling that in any TMSA run competitions if your instrument has eight or less bass buttons, your playing a melodian, if your instrument has more that eight bass buttons your playing a button accordion.