How to Use Id. and Supra in Legal Citations
Two of the most frequently used short citation forms in legal writing are Id. and supra. Getting them right is essential for law review, seminar papers, and any academic legal writing. This guide explains when and how to use each one, with clear rules and real examples.
What Is Id.?
Id. is an abbreviation of the Latin word idem, meaning "the same." In legal citation, it tells the reader that you are referring to the same source cited in the immediately preceding citation. It is the shortest and most efficient way to repeat a citation in legal writing.
Id. is pronounced "idd" (rhyming with "lid"), not "eye-dee." It is always italicized, always followed by a period (because it is an abbreviation), and always capitalized when it begins a citation sentence. When it appears as part of a citation clause within a sentence, it is not capitalized.
The Bluebook governs Id. usage primarily in Rule 4.1. It is the most common short citation form you will encounter in law review footnotes and academic legal writing.
When to Use Id.
The rule is straightforward: use Id. when you are citing the exact same source as the one in the immediately preceding citation. The key word is "immediately." If the citation directly before yours — whether in the same footnote or the previous footnote — cites only one source, you may use Id. to refer to it.
Footnote 5
Miranda v. Arizona, 384 U.S. 436, 444 (1966).
Footnote 6
Id. at 467.
Footnote 7
Id.
In this example, footnote 6 uses Id. at 467 because it refers to the same case (Miranda) but a different page. Footnote 7 uses Id. alone because it refers to the same source and the same page as footnote 6.
Id. vs. Id. at [page]
The distinction is simple but important. Use Id. alone when you are citing the exact same source and the exact same pinpoint page as the immediately preceding citation. Use Id. at [page] when you are citing the same source but a different page or section.
Same source, same page
Id.
Same source, different page
Id. at 512.
Same source, different section (statute)
Id. § 302(b).
Note the format carefully: there is a space between "Id." and "at." The word "at" is not italicized — only "Id." is. For statutes and other sources that use section symbols or paragraph markers instead of page numbers, substitute the appropriate symbol for "at [page]."
If the preceding citation had no pinpoint and you now want to add one, use Id. at [page]. If the preceding citation had a pinpoint and you want to refer to the same page, use Id. alone.
When NOT to Use Id.
There are several situations where Id. is not appropriate, even if one of the sources in the preceding citation is the same source you want to cite:
Multiple sources in the preceding footnote
If footnote 12 cites three different sources, footnote 13 cannot use Id. because the reader will not know which of the three sources you mean. Use a short form citation instead.
String citations
In a string citation (multiple authorities cited together), do not use Id. to refer to one of the other sources in the same string. Each source in a string citation should be independently identifiable.
Cross-referencing across many footnotes
If the full citation appeared twenty footnotes ago and you have cited other sources since, Id. cannot be used. Use the short form or supra instead.
The underlying principle is clarity. Id. works only when there is zero ambiguity about which source it refers to. If there is any doubt, use a more specific short form.
What Is Supra?
Supra is Latin for "above." In legal citation, it directs the reader to a source that was fully cited in an earlier footnote. Unlike Id., which refers only to the immediately preceding citation, supra can reach back to any earlier footnote.
There is one critical limitation: supra cannot be used for cases, statutes, constitutions, or regulations (Bluebook Rule 4.2). For those primary legal authorities, you must use the standard short form citation (e.g., Miranda, 384 U.S. at 448). Supra is reserved for secondary sources — law review articles, books, reports, treaties, and similar materials.
Full citation (footnote 3)
Cass R. Sunstein, On Analogical Reasoning, 106 Harv. L. Rev. 741, 745 (1993).
Supra citation (footnote 18)
Sunstein, supra note 3, at 760.
The format is: Author last name, supra note [number], at [page]. The word "supra" is always italicized. The note number refers to the footnote where the full citation first appeared.
Supra Format for Different Source Types
The basic supra format varies slightly depending on the type of source. Here are the most common formats you will encounter in law school:
Law Review Article (single author)
Sunstein, supra note 3, at 760.
Law Review Article (multiple authors)
Goldberg & Zipursky, supra note 12, at 1742.
Book (single author)
Posner, supra note 7, at 145.
Report or institutional author
Am. Bar Ass’n, supra note 22, at 18.
Treatise (with volume)
2 Wigmore, supra note 9, § 285.
For works with more than two authors, use the first author's last name followed by "et al." For institutional authors, use the abbreviated name as it appeared in the full citation. If the work is commonly known by its title rather than its author, you may use the title in the supra reference instead.
Hereinafter — When and How to Use It
The hereinafter designation allows you to create a shortened name for a source that would otherwise be cumbersome to cite repeatedly. It is defined in the first full citation and then used with supra in subsequent references.
Use hereinafter when the author's name or the title of the work is very long, when citing a source that does not have a clear author, or when two works by the same author are cited in the same piece and you need to distinguish them.
Full citation with hereinafter (footnote 4)
Nat’l Conf. of Comm’rs on Unif. State Laws, Uniform Commercial Code § 2-302 (Am. L. Inst. & Nat’l Conf. of Comm’rs on Unif. State Laws 2022) [hereinafter UCC].
Subsequent reference (footnote 19)
UCC, supra note 4, § 2-615.
The hereinafter label appears in square brackets at the end of the first full citation. In all subsequent citations, use the label as the identifier before supra. The label should be short, distinctive, and intuitive — the reader should be able to guess what source it refers to.
Id. vs. Supra vs. Short Form — A Decision Flowchart
Choosing the correct short citation form can feel confusing at first. Use the following decision process to determine which form to use:
Is this the first time you are citing this source? If yes, give the full citation. Stop here.
Is the immediately preceding citation to this same source (and only this source)? If yes, use Id. (or Id. at [page] for a different page).
Is the source a case, statute, constitution, or regulation? If yes, use the standard short form (e.g., Miranda, 384 U.S. at 448). Do not use supra.
Is the source a secondary source (article, book, report)? If yes, use supra (e.g., Sunstein, supra note 3, at 760).
Is the source name unwieldy? Consider adding a [hereinafter] designation in the full citation so future supra references are cleaner.
In summary: Id. is for the immediately preceding source regardless of type. Supra is for secondary sources cited earlier but not immediately preceding. Short forms are for primary legal authorities cited earlier but not immediately preceding.
Common Mistakes with Id. and Supra
Using Id. when the preceding footnote cites multiple sources
This is the single most common Id. error. If footnote 10 cites a case and a law review article, footnote 11 cannot use Id. to refer to either one. Use the appropriate short form for whichever source you mean.
Using supra for cases or statutes
Supra is never used for cases, statutes, constitutions, or regulations. For these primary sources, always use the standard short form (one party name, volume, reporter, "at" page).
Forgetting to italicize Id. or supra
Both Id. and supra are always italicized in Bluebook format. The word "at" following Id. is not italicized, and "note" following supra is not italicized.
Omitting the period after Id.
Id. is an abbreviation, so it always has a period. Writing "Id at 450" instead of "Id. at 450" is incorrect. The period is part of the abbreviation, not the end of the citation sentence.
Using the wrong footnote number with supra
Supra note [n] must refer to the footnote where the full citation first appeared, not some later footnote where a short form was used. Double-check your note numbers when editing.
Capitalizing Id. mid-sentence when it should be lowercase
Id. is capitalized only when it begins a citation sentence. When used in a citation clause within a sentence, it should be lowercase: see id. at 450.