26 C.F.R. § 1.661(b)-1
In the absence of specific provisions in the governing instrument for the allocation of different classes of income, or unless local law requires such an allocation, the amount deductible for distributions to beneficiaries under section 661(a) is treated as consisting of the same proportion of each class of items entering into the computation of distributable net income as the total of each class bears to the total distributable net income. For example, if a trust has distributable net income of $20,000, consisting of $10,000 each of taxable interest and royalties and distributes $10,000 to beneficiary A, the deduction of $10,000 allowable under section 661(a) is deemed to consist of $5,000 each of taxable interest and royalties, unless the trust instrument specifically provides for the distribution or accumulation of different classes of income or unless local law requires such an allocation. See also § 1.661(c)-1 .
26 C.F.R. §1.661(b)-1