17 CFR 18.01: Interest in or control of several accounts.

If any trader holds, has a financial interest in or controls positions in more than one account, whether carried with the same or with different futures commission merchants or foreign brokers, all such positions and accounts shall be considered as a single account for the purpose of determining whether such trader has a reportable position and, unless instructed otherwise in the special call to report under § 18.00 for the purpose of reporting.