26 CFR § 1.307-1 - General.
(a) If a shareholder receives stock or stock rights as a distribution on stock previously held and under section 305 such distribution is not includible in gross income then, except as provided in section 307(b) and § 1.307–2, the basis of the stock with respect to which the distribution was made shall be allocated between the old and new stocks or rights in proportion to the fair market values of each on the date of distribution. If a shareholder receives stock or stock rights as a distribution on stock previously held and pursuant to section 305 part of the distribution is not includible in gross income, then (except as provided in section 307(b) and § 1.307–2) the basis of the stock with respect to which the distribution is made shall be allocated between (1) the old stock and (2) that part of the new stock or rights which is not includible in gross income, in proportion to the fair market values of each on the date of distribution. The date of distribution in each case shall be the date the stock or the rights are distributed to the stockholder and not the record date. The general rule will apply with respect to stock rights only if such rights are exercised or sold.
(b) The application of paragraph (a) of this section is illustrated by the following example: