D.C. Code § 42-510
A future estate is one limited to commence at a future day, either without the intervention of a precedent estate or after the expiration or determination of a precedent estate created at the same time and by the same conveyance or devise.
Mar. 3, 1901, 31 Stat. 1351, ch. 854, § 1020
1973 Ed., § 45-810.
1981 Ed., § 45-210.