Idem

Id. (masculine and neuter) and ead. (feminine) (Latin, short for idem and eadem, "the same") denote the previously cited source (compare ibid.). Id. is particularly used in legal citations. They are also used in academic citations replacing the name of a repeated author. Id. is used extensively in Canadian legislation to apply a short description to a section with the same focus as the previous. Id. is an abbreviation where the last two letters of the word are not present; thus, it always takes a period (or full stop) in both British and American usage.

Legal example

Here, the first citation refers to the case of United States v. Martinez-Fuerte. The volume number cited is 428 and the page on which the case begins is 543, and the page number cited to is 545. The "U.S." between the numerical portions of the citation refers to the United States Reports. 1976 refers to the year that the case was published. The second citation references the first citation and automatically incorporates the same reporter and volume number; however, the page number cited is now 547. Id. refers to the immediately preceding citation, so if the previous citation includes more than one reference, or it is unclear which reference Id. refers to, its usage is inappropriate.