I remember that the practice of using a second, cross-guaranteed account to put on a "box" position for use in shorting on downticks was outlawed some years ago. Was it the entire practice of boxes, or just using two accounts that shared the same capital? That is, can I legally (i.e. according to SEC/NYSE rules) set up two independent accounts, at two firms, and carry equivalent long and short positions in the two accounts? Yes, it's margin inefficient, but cheaper and cleaner than buying (and renewing) a conversion.