One of the first things you notice when you look at a typical case opinion in a book (or an opinion printed from Westlaw) is a collection of numbered headnotes at the top.
In any reporter published by Thomson or West, these are West Key Numbers and headnotes. Attorneys and editors at Thomson/West read the case, decided what the important holdings were, and then added these notes to the published version of the case. This links the case to the larger Digest System, which Thomson/West uses to classify every published case in the country by subject.
Over 100 years ago, West Publishing Company divided the entire body of American law into a discrete set of "topics," broke each topic down into hundreds of individual elements, and then assigned each of those elements a "key number." Over the years, some topics have been added, others divided or their elements rearranged, but the basic system hasn't changed (also, the little picture of the key provided a nifty logo for West's books!). Take a look at the front of any digest book to see how the publisher divided up American law into topics. When, in the 1970's, West introduced its "Westlaw" computer databases, it assigned each topic a number as well; these can be difficult to find, but it's worth it to do so, if you want to use Westlaw to find more cases. More about that later.
There are two typical ways that the digest system is used:
For example, your firm has a discrimination case in federal court. You want to find cases saying that, even where most sexually harassing behavior in a workplace happened well before the statute of limitations for Title VII actions, if some harassment happened so recently as to be within the statute of limitations period, the plaintiff can sue and recover for all of the harassment, no matter how long ago it happened. This is called the "continuing violation theory." You have found a helpful case from the federal Eighth Circuit Court of Appeals in Minnesota on this issue:
Jenson v. Eveleth Taconite Co., 130 F.3d 1287 (8th Cir. 1997). (Interestingly, this case was the basis of the movie "North Country," with Charlize Theron. Sadly, she will not appear in this guide.)
But you need to find law in the Ninth Circuit, because your case is filed in the federal court for the Southern District of California. How do you get from your Minnesota case to a case that is useful in California (or, hopefully, more than one)?
Look at the headnotes at the beginning of the Jenson case (you can click on the picture at left to see the entire case). Headnotes 19 through 23 address your issue. The key numbers associated with those headnotes are: Civil Rights (key) 342, Civil Rights (key) 373, and Civil Rights (key) 448.1. Make sure that you read the actual text of the opinion (that is, the part the court wrote) after the numbers  -  in the main part of the case, to make sure that you agree that the headnotes correctly reflect the holding of the court, and that the case really is helpful to you. (And, by the way, if you quote from a case, make sure you quote from the actual opinion, not the headnotes. It is very embarrassing to be discovered quoting language that was not actually written by a judge.)
Now you need to go to the correct digest. West has papered the country with digests, and the cases you want will be "digested" in more than one. There are:
State Digests (for example, West's California Digest, which digests all published federal and state cases which are authority for California courts);
Regional Digests (for example, West's Pacific Digest, whichdigests all the cases reported in the Pacific Reporter. This reporter includes not just the obvious Pacific states of Alaska, California, Hawai'i, Oregon and Washington, but also, bafflingly, Arizona, Colorado, Idaho, Kansas, Montana, Nevada, New Mexico, Oklahoma, Utah and Wyoming. Perhaps they are preparing for when California falls into the sea.);
West's Federal Practice Digest, which digests all published federal court cases;
The Decennial Digests, which digest every published case in the country, and are issued every ten years; and
The General Digest, which is issued after the most recent decennial digest, to update it. (These are quite a slog to use. Avoid them unless your library has nothing else which covers your jurisdiction.); various specialized digests, such as West's Bankruptcy Digest; and, for something really obscure,
The Century Edition of the American Digest, which digests cases issued between 1658 and 1896.
In this case, because you are searching for cases which interpret a federal law (Title VII) and are binding authority in the federal courts of California,
West's Federal Practice Digest is probably the best digest for you. So, take your key numbers to the most recent edition of West's Federal Practice Digest (which happens to be the 4th). Look for the book(s) containing your topic ("Civil Rights") and your key numbers. Wait! Your key numbers don't seem to exist. Was there some huge typographical error? No, Civil Rights is one of those topics which has exploded with new and interesting developments over the past twenty years, and as a result was completely renumbered in 2003.
Find the "Key Number Translation Table" right after the table of contents for the topic, and convert your old key numbers into new, improved key numbers. Click on the picture at left to see what the first page of this table looks like. Key number 343 becomes 1505, 373 becomes 1530, and 448.1 becomes (yikes!) 1717, 1723-25, 1727-1733, and 1746-1758. Probably best to look in the Civil Rights table of contents for these key numbers, and see which looks the most useful. Here, it's probably Civil Rights (key) 1505(7), which is described as "Time for proceedings; limitations; Continuing violations; serial, ongoing, or related acts."
Now you can look at the case annotations under this key number in the digest, and see cases from throughout the federal court system on the same subject. Click on the picture at left to see the first page of these case annotations. Here, the first case listed under Civil Rights (key) 1505(7) is Nat'l R.R. Passenger Corp. v. Morgan, in which the Supreme Court held: " We conclude that a Title VII plaintiff raising claims of discrete discriminatory or retaliatory acts must file his charge within the appropriate time period -- 180 or 300 days -- set forth in 42 U.S.C. sec. 2000e-5(e)(1). A charge alleging a hostile work environment claim, however, will not be time barred so long as all acts which constitute the claim are part of the same unlawful employment practice and at least one act falls within the time period ." Nat'l R.R. Passenger Corp. v. Morgan, 536 U.S. 101, 122 (2002)(emphasis added). Sounds helpful. But even though this is a Supreme Court case, make sure that you Shepardize or Keycite it to make sure that it is still good law, and applies to your type of case in California. And whatever you find in the book, don't forget to look in the pocket part inside the back cover (or the paperback book following the volume, if it has grown too big to be a pocket part) and the supplement pamphlets at the end of the entire set, for even more recent cases.
This means that you need to figure out for yourself which key numbers to look under. Let's say you have a case in which one party to a telephone conversation recorded that conversation without telling the other party to the conversation, and then sold the recording to a tabloid newspaper. Both parties were in California. Can the injured party sue for damages?
This time West's California Digest 2d seems like the best place to look. Look in the Descriptive Word Index books to find the correct topic and key number. For example, the index has lots of entries under "Telephones," including "Conversations; Interception. See heading Electronic Surveillance or Interception, generally." A look at that heading, in turn, leads you to "Civil liability, Tel (key) 498; See also heading TORTS, PRIVACY, invasion of." "Tel" means "Telecommunications"; key number 498 provides an interesting collection of cases which may be helpful. A look at the pocket part to the book shows that -- not again! -- the Telecommunications topic has been renumbered, and 498 has now become 1253, 1436, 1441, 1443 and 1451. But the new table of contents for Telecommunications shows that one of these key numbers is for cable television cases, and so is probably not useful here. Key numbers 1436 - 1451, though, provide some more helpful cases. According to the table of contents for the Torts topic, 8.5(5) is the key number for "Invasion of privacy; Publications or communications." In the supplement, this topic, too, has been renumbered, and 8.5(5) has become 350-54.
If this seems a bit indirect, you can also try looking for a discussion of your legal issue in California Jurisprudence or Corpus Juris Secundum; both of these, as Thomson/West publications, include references to the appropriate key numbers for each topic -- and will also probably give you some useful cases.
1) The Table of Cases for each digest allows you to find a citation for a case when all you know is the name. For example, if you have heard of the "Jenson Case" in some federal court, and know that the name of the other party has the word "taconite" in it (just what is taconite, anyway?), you can look under "Jenson" in the Table of Cases for the Federal Practice Digest and find the case easily. If you are not sure just where the case took place, you may need to look in the Tables of Cases of several different digests. This is particularly handy if you don't have access to Westlaw or Lexis, or some other online research tool.
2) The Words and Phrases volumes of each digest allow you to find cases interpreting particular terms. For example, you need to quickly find a case to cite for the definition of the obvious word "contract." Honestly, you learned this in your first week of Contracts, didn't you, and yet where is an actual California case that you can cite in court? Look the word up in West's California Digest, and you will find cases which you can cite. But remember: READ THE ACTUAL CASE before you cite it. You and the attorneys at West may not agree, and the opinion may have other details which are bad for your case.
The term "key number" is owned by Thomson/West, but many other publishers have developed their own digesting systems, often for specific subjects, like tax law, or specific sets of books, like the Lawyers' Edition (L. Ed.) Supreme Court reports.
In addition, until 2003 the digest of California's official case reports was McKinney's New California Digest, which used its own numbering and headnote system. Bancroft-Whitney discontinued this digest when the contract for publishing the official reports was awarded to another publisher.
Finally, Lexis-Nexis has developed its own headnote system, and is busily adding headnotes to many of the cases in its online databases. To see a table of these, click on the "Search Advisor" tab on your Lexis screen. And as with Westlaw, if you locate a case online which has a useful Lexis headnote, you can then search for cases with this headnote in other jurisdictions, although the Lexis headnote system is not as detailed as West's.