26 USC 2036 - Transfers with retained life estate

(a) General rule 
The value of the gross estate shall include the value of all property to the extent of any interest therein of which the decedent has at any time made a transfer (except in case of a bona fide sale for an adequate and full consideration in money or moneys worth), by trust or otherwise, under which he has retained for his life or for any period not ascertainable without reference to his death or for any period which does not in fact end before his death
(1) the possession or enjoyment of, or the right to the income from, the property, or
(2) the right, either alone or in conjunction with any person, to designate the persons who shall possess or enjoy the property or the income therefrom.
(b) Voting rights 

(1) In general 
For purposes of subsection (a)(1), the retention of the right to vote (directly or indirectly) shares of stock of a controlled corporation shall be considered to be a retention of the enjoyment of transferred property.
(2) Controlled corporation 
For purposes of paragraph (1), a corporation shall be treated as a controlled corporation if, at any time after the transfer of the property and during the 3-year period ending on the date of the decedents death, the decedent owned (with the application of section 318), or had the right (either alone or in conjunction with any person) to vote, stock possessing at least 20 percent of the total combined voting power of all classes of stock.
(3) Coordination with section 2035 
For purposes of applying section 2035 with respect to paragraph (1), the relinquishment or cessation of voting rights shall be treated as a transfer of property made by the decedent.
(c) Limitation on application of general rule 
This section shall not apply to a transfer made before March 4, 1931; nor to a transfer made after March 3, 1931, and before June 7, 1932, unless the property transferred would have been includible in the decedents gross estate by reason of the amendatory language of the joint resolution of March 3, 1931 (46 Stat. 1516).