26 C.F.R. § 7.465-1
Section 465 provides that a taxpayer (other than a corporation which is not a subchapter S corporation or a personal holding company) engaged in certain activities may not deduct losses from such activity to the extent the losses exceed the amount the taxpayer is at risk with respect to the activity. For the types of activities to which section 465 applies and for determining what constitutes a separate activity, see section 465(c). Section 465 generally applies to losses attributable to amounts paid or incurred in taxable years beginning after December 31, 1975. For the purposes of applying the at risk limitation to activities begun before the effective date of the provision (and which were not excepted from application of the provision), it is necessary to determine the amount at risk as of the first day of the first taxable year beginning after December 31, 1975. The amount at risk in an activity as of the first day of the first taxable year of the taxpayer beginning after December 31, 1975, (for the purposes of § 7.465-1 through 7.465-5 such first day shall be referred to as the effective date) shall be determined according to the rules provided in §§ 7.465-2 through 7.465-5 .
26 C.F.R. §7.465-1