Hi All, I'm relatively new to options and have been reading the various 'Options' threads with great interest. Amongst other things, I've learned that a vertical debit spread can bring with it the risk of a margin call should the short side temporarily be more valuable than the long (http://www.elitetrader.com/vb/showt...t=vertical and spread and margin&pagenumber=1). So, before I even think of trading, I had a couple questions of my own: 1) How often does the sort of temporary pricing imbalance mentioned in the cited thread actually occur? Is this relatively common, and does it typically trigger a margin call no matter the broker you use? 2) Assume that I have a credit vertical call spread of 10 contracts on the ever-popular company XYZ: Sell Mar 50 Call @2.50 Buy Mar 55 Call @1.50 Net credit of $1.00, which is applied to the $5 maintenance margin requirement for a net maintenance requirement of $4000. Okay, assume underlying moves to 52 and I'm assigned (before expiration) on the short 50. Now I have a couple of choices: 1) Go out into the market and buy the shares to cover the short stock position. Hold the 55 calls, or sell them for whatever time premium I can get. Or... 2) Exercise the 55 calls. Now, you may think, why on earth would you exercise the long calls when you are going to pay $3 above current share price? And thus my question: In choice (1) I must have sufficient cash to be able to purchase the stock to close my short position. However--if I understand this correctly--if I exercise my long calls as in choice (2) within one business day of assignment, I need not post additional margin, and the long call exercise offsets the short shares, netting a loss of $3/share, to be sure, but not requiring me to cough up $52,000 in cash to buy the shares outright. Is this a correct summation, and is this 'same day substitution'? I realize that this is probably pathetically simple stuff to you, but as a newbie I want to make sure I understand what my potential choices are and what costs are associated with each. Thanks in advance for any insights you can offer!