Citation Crosswalk for Parallel Citations

About Parallel Citations

In general, legal citations are created when case law is printed in a physical book. When that happens, you can pull a volume off a shelf, find a decision within it, and check the page where that decision is printed. For example, if you pull volume 135 of the Supreme Court Reports off the shelf and turn to page 2,584, you'll find Obergefell v. Hodges.

This is problematic because unless you scan thousands of books or scrape publisher websites (in violation of their terms of service), you cannot get the citations. Some legal decisions are published in multiple books, which leads to what are called "parallel citations" — multiple citations for the same decision, one from each book.

It gets worse. There are also online-only citations like WL and LEXIS, which are used by online platforms to denote unpublished case law that doesn't get printed in books.

And finally, a third type of citation is a neutral citation. These are created when courts create and publish citations for their legal decisions as they adjudicate cases. These citations are non-proprietary and easily accessed on court websites. Long term, neutral citations are the way out of this problem. We urge courts to adopt them.

The net of this is that a single legal decision can have many "parallel" citations to various books, websites, and court citations. It is difficult to collect all these citations, and nearly impossible to figure out which ones refer to the same decisions.

Our Solution

We have spent over a decade collecting citations and have launched a case law scanning project to gather citations from books as they are published.

To date, we have gathered 18,121,982 citations, which we make available on a quarterly basis as a bulk download called citations-$DATE.csv.bz.

This is a CSV that has one row for each citation in our system. The columns are for the volume, reporter abbreviation, page, and decision ID. For example, Obergefell, above, has the following parallel citations:

576 U.S. 644
135 S. Ct. 2584
192 L. Ed. 2d 609
2015 U.S. LEXIS 4250

So in the CSV you get:

576  | U.S.       | 644   | 2812209
135  | S. Ct.     | 2584  | 2812209
192  | L. Ed. 2d  | 609   | 2812209
2015 | U.S. LEXIS | 4250  | 2812209

To crosswalk these citations, simply group by the last column, and you'll have all the citations we know for a given case.