(1) In this Part, as it applies for income tax purposes, "basis period" has the meaning assigned to it by this section.

(2)

(a) Subject to paragraph (b), in the case of a person to whom an allowance or on whom a charge is to be made under Case I of Schedule D in charging the profits or gains of the person's trade or under Case V of Schedule D in charging income arising from rents or receipts in respect of premises or easements, the person's basis period for any year of assessment shall be the period on the profits or gains of which income tax for that year is to be finally computed under Case I of Schedule D in respect of the trade in question or, as the case may be, under Case V of Schedule D in respect of the income arising from rents or receipts in respect of premises or easements or, where by virtue of the Income Tax Acts the profits or gains or income of any other period are to be taken to be the profits or gains or income of that period, that other period.

(b) In the case of any trade -