A private foundation, in the United States, is a charitable organization described in the Internal Revenue Code by section 509. A private foundation is necessarily a 501(c)(3) exempt organization (or a former such entity). It is defined by a negative definition: by what it is not. A private foundation is not a public charity, as described in section 170(b)(1)(A) (i) through (vi). Neither is it a section 509(a)(2) organization, nor a supporting organization. Private foundations are subject to 2% excise taxes found in section 4940 through 4945 of the internal revenue code. Once a charity becomes a private foundation, it retains that status unless it follows the difficult termination rules of section 507.