Not quite right. There's the DTC (which carries out 95% of the trades), then there's clearing houses (sometimes a seperate entity, sometimes part of the broker: the larger ones all do their own clearing. e.g. Robinhood has their own clearing house), then there's the brokers, then the traders. The DTC said 'we need 100% collateral on these trades' (probably because they viewed there being a significant risk that someone, somewhere, would not be able to pay up for the trades they're making in a big way), and then APEX clearing (which is the clearing house used by a lot of smaller brokers but not 95% of the market) said 'we can't support opening new positions' because their liquidity would be sucked up by the requirement, blocking most of the smaller brokers. Robinhood did similar for similar reasons independently of APEX, but some other brokers not using APEX could still trade (either because they had more cash on hand or because their customers weren't buying as much of affected stocks).