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My try at diagramming what David and JB are describing (hope I did this right)...

192211
(click pic enough times to make it big enough to see)

Well, if you're alright with a B7 instead of B - and if you're using it to lead to an Em chord, why wouldn't you be - then just leave the A string open. ;)

I have to say, that's a bear of a chord...

A bit loose perhaps but should be Ok
12p 18w 30w 45w is a nice set and what i use or 13 20 30 46 if you want a bit more tension but it might depend on your guitars make and model and your playing...