Butterfly badge stock is a stock that has a perforated fold line in the centre of the stock and has a sticky back. When printed, the two halves of the badge are folded and stuck together to create a double-sided badge.
The butterfly stock is a different size to a standard A6, even when folded in half:
Butterfly stock - 97mm x 269mm is a portrait badge, which is slightly smaller than a standard A6 when folded.
Butterfly stock - 102mm x 164mm is a small landscape badge. When folded, it is roughly half a standard A6.