26 CFR § 1.651(a)-1 - Simple trusts; deduction for distributions; in general.
Section 651 is applicable only to a trust the governing instruments of which:
and does not make any distribution other than of current income. A trust to which section 651 applies is referred to in this part as a “simple” trust. Trusts subject to section 661 are referred to as “complex” trusts. A trust may be a simple trust for one year and a complex trust for another year. It should be noted that under section 651 a trust qualifies as a simple trust in a taxable year in which it is required to distribute all its income currently and makes no other distributions, whether or not distributions of current income are in fact made. On the other hand a trust is not a complex trust by reason of distributions of amounts other than income unless such distributions are in fact made during the taxable year, whether or not they are required in that year.