S6166_Paragraph
(b1) - Section 6166(b)(1) - Interest in a Closely Held Business
Section 6166(b)(1)(C)  - Example 11
Decedent owned 30% of the voting stock in Corporation C and 40% of the voting stock in Corporation D. Both corporations have more than 45 shareholders, none of which are §267(c)(4) family members of the Decedent. More than 20% in value of the voting stock in each corporation is included in the Decedent's gross estate, satisfying the requirements of §6166(b)(1)(C)(i). The estate tax values of Decedent's interests in the Corporation C stock and the Corporation D stock exceed 35% of the §6166(b)(6) adjusted gross estate.
The estate wishes to combine the estate tax values of each Corporation to be treated as an interest in a single Corporation under §6166(c). Section 6166(c) requires that 20% or more of the total values of Corporation C and Corporation D must be included in Decedent's gross estate.
However, Corporation C has 3 classes of stock, only one of which is voting stock. The estate tax value of Decedent's interest in Corporation C is less than 20% of the total value of Corporation C, even though it is more than 20% in value of the voting stock.
Result:  The estate tax values of Decedent's interests in Corporations C and D cannot be combined to be treated as an interest in a single closely held business for purposes of §6166 even though the requirements of §6166(b)(1)(C)(i) have been satisfied. Only the tax attributable to Corporation D may be extended under §6166, but not the tax attributable to both.
However, the estate could elect to bifurcate the tax extended under §6166. Section 6166(c) would not be applicable. With bifurcation, it is enough that each closely held business interest satisfies the requirement of §6166(b)(1)(C)(i) and that each value for determining the Decedent's gross estate exceeds 35% of the §6166(b)(6) adjusted gross estate. The estate tax attributable to each closely held business would independently qualify for a separate 14-year §6166(a) extension. Each §6166 account should be separately maintained by IRS for billing purposes (bifurcation accounts have been established at Cincinnati Campus for a number of estates.)
Section 6166(b)(1)(C)  - Example 11 
§6166(c) Election Requires a §6166(b)(7) Election
Corporation C - 50 Shareholders
Stock ClassShares OutstandingShares Owned by Decedent
Voting common10,0003,000
Non-voting common10,0000
Non-voting preferred1,0000
Corporation D - 60 Shareholders
Voting common10,0004,000
Corporation C - §6166(b)(1)(C)(i) 20% Test
Full value of Corporation C voting common stock50,000,000.00
Outstanding shares of voting common stock10,000
Per-share value of Corporation C voting common stock5,000.00
Valuation discounts22%
§6166(b)(4) discounted estate tax value per share3,600.00
Shares of Corporation C voting common stock owned by Decedent at date of death (30% of the total outstanding)3,000
§6166(b)(4) estate tax value of Decedent's 3,000 shares of voting stock of Corporation C11,700,000.00
Percentage in value of Corporation C voting common stock included in determining Decedent's gross estate - the 20% test of §6166(b)(1)(C)(i) is satisfied. However, the 20% test of §6166(c) is not satisfied (see below).23.40%
§6166(b)(6) Adjusted Gross Estate Computation
Gross estate38,300,000.00
Sections J, K, and L deductions - §2053 and §20545,250,000.00
§6166(b)(6) adjusted gross estate15,590,000.00
§6166(a)(2) Ratio Computation
Section 6166 closely held business amount - Corporation C alone11,700,000.00
Section 6166(b)(6) adjusted gross estate33,050,000.00
Section 6166(a)(2) ratio - maximum amount of tax which may be paid in installments - qualifies for a §6166 extension - ratio is greater than 0.350000. However, the 20% test of §6166(c) is not satisfied (see below).0.354009
Corporation D - §6166(b)(1)(C)(i) 20% Test
Full value of Corporation D voting common stock45,000,000.00
Outstanding shares of voting common stock - 40% owned by Decedent10,000
Per-share value of Corporation D voting common stock4,500.00
Valuation discounts25%
§6166(b)(4) discounted estate tax value per share3,375.00
Shares of Corporation D voting common stock owned by Decedent at date of death (40% of the total outstanding)4,000
§6166(b)(4) estate tax value of Decedent's 4,000 shares of voting stock of Corporation D13,500,000.00
Percentage in value of Corporation D voting common stock included in determining Decedent's gross estate - the 20% test of §6166(b)(1)(C)(i) is satisfied30.00%
The §6166(c) 20% Determinations
Corporation C
Corporation C voting common stock full value50,000,000.00
Corporation C non-voting common stock full value35,000,000.00
Corporation C non-voting preferred stock full value15,000,000.00
Total value of Corporation C100,000,000.00
§6166(b)(4) estate tax value of Decedent's 3,000 shares of voting stock of Corporation C included in determining Decedent's gross estate11,700,000.00
Percentage of total value of Corporation C included in determining Decedent's gross estate - does not satisfy the 20% of total value test of §6166(c)11.70%
Corporation D
Total value of Corporation D45,000,000.00
§6166(b)(4) estate tax value of Decedent's 4,000 shares of voting stock of Corporation D included in determining Decedent's gross estate13,500,000.00
Percentage of total value of Corporation D included in determining Decedent's gross estate - satisfies the 20% of total value test of §6166(c)30.00%
The estate tax values of Decedent's interests in Corporation C and Corporation D cannot be combined to be treated as an interest in a single closely held business - the 20% test of §6166(c) is not satisfied as to Corporation C. The estate can file a regular §6166(a) 14-year election only for the estate tax attributable to Corporation D, as the estate tax value of Decedent's interest in Corporation C is less than 35% of the adjusted gross estate.
However, the estate could elect to bifurcate the tax extended under §6166. Section 6166(c) would not be applicable. With bifurcation, it is enough that each closely held business interest satisfies the requirement of §6166(b)(1)(C)(i) and that each value for determining the Decedent's gross estate exceeds 35% of the §6166(b)(6) adjusted gross estate. The estate tax attributable to each closely held business would independently qualify for a separate 14-year §6166(a) extension.
§6166(b)(6) Adjusted Gross Estate Computation
Gross estate38,300,000.00
Sections J, K, and L deductions - §2053 and §20545,250,000.00
§6166(b)(6) adjusted gross estate15,590,000.00
§6166(a)(2) Ratio Computation
Section 6166 closely held business amount - Corporation D alone13,500,000.00
Section 6166(b)(6) adjusted gross estate33,050,000.00
Section 6166(a)(2) ratio - maximum amount of tax which may be paid in installments - Qualifies for a §6166(a) 14-year Extension - ratio is greater than 0.350000.0.408472

Section 6166(b)(4) provides that for purposes of section 6166, "value" shall be the value determined for purposes of determining the Decedent's gross estate.