[charlesdm]

Interesting. I don't usually use margin since I live in a country without capital gains tax, but if I transfer $10k worth of shares and I want to use it as a line of credit, then I can at most take out $5k (in cash) before getting margin called?

[sokoloff]

Yes and no (mostly yes). The 50% level is the "initial margin" limit (hence the "yes" part) for initiating a loan.

There is a separate, lower limit called the "maintenance margin" limit, which is the equity percentage that you need to hold to avoid a margin call on a held position.

So, if you deposit $10K in shares, they appreciate to $12K, you borrow $6K and then the share price falls back to the original amount, you have $10K in shares, $4K in net equity (40%), but are probably not going to receive a margin call.

Or, if they are worth $10K, you borrow the limit of $5K and the share price falls by 1%, you won't get a margin call there either (on most securities).