Maybe a dumb question, but say I have $100 in account equity and I invest it all in T-bills. Then, using the $100 in T-bills for collateral I buy $100 in stock. Would the T-bills be used solely as margin collateral for the stock, or is the margin collateral requirement spread evenly across all of the securities held (or something like that)? What if the situation is reversed and I owned $100 in stock and then bought $100 of T-bills, would the stock be the collateral for the T-bills? How is it decided which assets are counted as collateral and which assets are controlled with the borrowed funds? Also, is it possible to choose which assets are the collateral and which are controlled with the borrowed money?