Section 6166(a)(1) provides:

(a) 5-year deferral; 10-year installment payment

(1) In general

If the value of an interest in a closely held business which is included in determining the gross estate of a decedent who was (at the date of his death) a citizen or resident of the United States exceeds 35 percent of the adjusted gross estate, the executor may elect to pay part or all of the tax imposed by section 2001 in 2 or more (but not exceeding 10) equal installments.

Reg. section 20.6166-1 provides:

§ 20.6166-1
    Election of alternate extension of time for payment of estate tax where estate consists largely of interest in closely held business.

(a) In general. Section 6166 allows an executor to elect to extend payment of part or all of the portion of the estate tax which is attributable to a closely held business interest (as defined in section 6166(b)(1)). If it is made at the time the estate tax return is filed, the election is applicable both to the tax originally determined to be due and to certain deficiencies. If no election is made when the estate tax return is filed, up to the full amount of certain later deficiencies (but not any tax originally determined to be due) may be paid in installments.