
The reason is simple enough to understand. BP shares are, say, cur-
rently 360p. Therefore 20 000 shares are worth £72 000; however, if
the option was assigned the writer would have to find 20 000 390
£78 000.
To be covered, either there would have to be more shares
deposited as collateral or the collateral would have to be ‘topped’
up with something additional like cash. Also the value of the collat-
eral must be monitored very carefully as the value could change
quickly and leave the margin call under-collateralised. For this rea-
son most brokers will agree with their client that the client should
leave