Companies cannot generally choose any accounting period in which to enter tonnage tax. FA 2000, Sch. 22, para. 10 provided rules concerning the initial entry ‘window’; this is called the ‘initial period’. It also provides rules for companies and groups that subsequently become eligible for tonnage tax for the first time. FA 2000, Sch. 22, para. 11 provides that the Treasury may appoint further entry ‘windows’ in future by the issue of regulations. See ¶790-220.