IB doesn't specifically mention the margin requirement for short calendars, but I imagine it is the same as short calls or puts. http://www.interactivebrokers.com/en/p.php?f=margin Comments?
I agree. If the requirement is to low the brokerage can lose money. If the requirement is to high the brokerage will lose customers. Butterfly's require less margin. If you buy the far month, and sell the near month, on a wide BF, wouldn't it serve the same ATM, w/ less risk. Carl