When chatting "Margin" remember that all Hedge Funds and all Quant Funds, who use margin to operate their trading accounts by depositing US Treasuries (just like TBTF banks and their oversized private derivatives), you have to keep in mind that all of the leveraged market players use what's now known as The Corzine Rule". Where all players leave absolutely no "Excess Margin" in the system, lest some unscrupulous banker use it to trade for his proprietary account, and lose it to JP Morgan through Tri-Party RePo claims, which is what Corzine was never prosecuted for when he tanked MF Global. (More on this later, because it means there is NO Cushion in any Institutional accounts when it hits the fan, and it becomes a bit systemic.)