Descriptive issues affecting statutes as subject headings
An important caveat to remember is that headings for laws are governed by AACR2 rather than LCSH, with the appropriate authority records residing in the Names Authority File, and that non-LC proposed headings arrive via NACO rather than SACO. Unlike topical subject headings, which are formulated based on LCSH and the Subject Cataloging Manual (which look to reference sources as authorities to determine usage), name authorities are formulated according to AACR2, and AACR2's rules are less than user friendly, particularly when used for a constructing a name authority heading for a statute.
The first element in the heading consists of the issuing jurisdiction. Sometimes a statute may be issued by one jurisdiction to be applied to another, such as the British North America Act which is an act of the British parliament that has served as Canada's de facto constitution. Until recently, all modern Scottish statutes were entered under Great Britain (though Scotland was used in the subfield Z of the 650 topical heading) and European Union legislation is never entered under jurisdiction since CPSO doesn't recognize the EU as a jurisdiction (it's a little more complicated than that, but de facto, that's the situation). There are deep philosophical meanings behind these decisions (e.g. did Scotland merge with England in 1707, or did it continue as a separate country whose legislative functions were outsourced to the merged parliament) which catalogers should avoid getting involved in.
Many significant Pakistani laws are still pre-independence Indian laws with anachronistic headings such as "Pakistan. Oath Act, 1873" (hint: there was no such thing as Pakistan 130 years ago). Indian cases before the breakup of imperial India are still "good law" in Pakistan, and books cite the law's treatment in the other country's court (since it is the same statute). Anyone desiring to preserve their sanity should avoid dealing with LC's treatment of "Palestinian" law over the course of the last 100 years (great subject for law review articles, bad subject for catalogers).
While these types of issues almost never arise with United States materials, they can confuse anyone working with "foreign" law. The form established for the name of the law can also create problems for access. LCSH establishes headings based on usage in English reference sources. AACR2 establishes its headings based on the actual publications in the country of origin. The names of administrative agencies (issuing bodies for regulations) and the headings for statutes are therefore in the "vernacular" (in the original language, rather than English), though the jurisdiction is established based on the English form of name. English language reference sources routinely speak of Israel's "Law of Return", AACR2 dictates a heading of "Israel. Hok ha-Shevut" (which of course is no problem if there are proper 4XX headings integrated into the OPAC, which of course all OPACs have and are fully understood by users, right?) Another complication is that, unlike American laws with "short titles" that usually reflect the nature of the law, many civil law countries use mechanical formulas such as "Italy. Decreto del presidente della Repubblica 18 luglio 1986. n. 545 " (i.e. Presidential decree of July 18, 1986, no. 545). Such headings are easy to construct for catalogers but worthless unless you already know which law you are looking for.
When is a heading for a law not needed or not useful?
Regardless of whether Memo H1715 allows you to use a heading for a statute, there are times when it isn't needed at all, and making one misleads the users. In some areas of law, the statute and the subject are coextensive. This occurs when the legislature (which can mean the Congress, the Queen in Council, or the chief warlord who unilaterally enacts laws since he is the boss—we don't make value judgments) decides to consolidate all the law on the subject, repeal all other laws (that which wasn't reenacted is repealed), and amends the consolidated law rather than passing supplemental new statutes. "New York. Penal Law" is frequently discussed in titles, but never has warranted a 610 heading at LC (though if it were more recent, a legislative history or a book on its enactment would warrant a 610). The 650 heading "Criminal law—New York (State)" covers the same material. All books on criminal law in New York are on the Penal code, and any book on the Penal code covers almost the entirety of New York criminal law. Common law crimes or pre-codification criminal statutes haven't had legal status in New York in living memory. In other states, such as Maryland, where common law crimes co-exist with statutory crimes, "Criminal law—Maryland" is by definition broader than any heading for a statute, and any book on a statute is a focused book on an aspect of Maryland criminal law, but never the totality. Unlike New York where new statutes typically amend the codified law (meaning after a year, one usually cites only the codified law), new statutes in Maryland frequently supplement existing statutes (and need to be cited).
The Federal Bankruptcy Law (NAF: United States. Bankruptcy) is another example. It is a section of United States Code, which includes all Federal legislation on the subject, and has been enacted into positive law (as opposed to other sections of USC which are quasi-official editorial arrangements of existing statutes). Therefore, there is never a need for a subject heading for the law, (except for a legislative history of its passage), since the heading "Bankruptcy—United States" covers the same territory. Of course, an individual act that amends the Bankruptcy law would warrant a heading, but after a year no one cares much about those laws, only the total law which assimilated them.
Most major codes in Roman law systems are similar to New York's Penal law or the Federal Bankruptcy law. All of "Civil law—[place]" is in the "[Place]. Civil code." and vice versa. Even though the 1995 changes to SCM allow a 610 heading for code, a 610 isn't too useful for those jurisdictions since every book about civil law is about the code. Indeed the term "Civil law" is defined by whatever Justinian included in Corpus juris civilis (the original and enduring paradigm for codification). While legislative history or a detailed textual analysis would still need a heading, even under the pre-1995 rule, unless it is a legislative history or a book on a new code, a 610 heading tells the user nothing they don't know from the 650. One should remember that the enactment of a new code in Roman law systems is a major event, e.g. the last country using Justinian's code didn't adopt a new one until the 20th century. The major codes are expected to last for generations. Of course many publishers create their own "code" out of collected statutes, but these codes are mere compilations and do not require any special consideration in subject headings.
Landmark legislation in the United States is often meaningless as a subject heading. Everyone who deals with Labor law knows about the Wagner Act and the Taft-Hartley Act, which LC of course refers to by their official names: "National Labor Relations Act" and "Labor-Management Relations Act, 1947". Official documents of the National Labor Relations Board routinely invoke them, but they are almost never used as subject headings. If you look at the text in United States Code, the reason is apparent. The unofficial codification weaves the two together (not enacted in positive law) so that one can not really be considered without the other, along with subsequent legislation that amends, replaces or affects both. Similarly, the Civil Rights Act of 1964 is rarely the subject of a book aimed at non-historical researchers since the law has been reworked so many times that a book on "United States. Civil rights act of 1964" is of little value to a user who wants to know the current status of "Civil rights—United States." In these situations, anything other than a legislative history of the original law does not need a subject heading for the statute, and in fact such a heading would mislead the users. No one writes books on the statute for legal practitioners, even if they include the statute in the title, rather they write on the area of law.
Constitutions and treaties
Even though constitutions and treaties are considered part of the law in the United States, AACR2 treats them very differently, and LCSH treats them somewhat differently.
A constitution (discussed in SCM H1465) raises the same issues as statutes. For the United States, "[Place]. Constitution" is coextensive with "Constitutional law—[Place]. The United States has had only one constitution and the United States adopted its own constitution (entering "Articles of confederation" under United States is slightly anachronistic since at the time it was a multilateral treaty that we now see as a de facto constitution). Only for a partial heading (a single amendment or group of amendments) would an author-title heading be of use for anyone researching American constitutional law. However the 610 headings would definitely help users in a country with a history of multiple constitutions (which includes most jurisdictions other than the United States, as well as most American states) or whose constitution incorporates multiple documents such as Israel and Great Britain.
Treaties (discussed in SCM H2227) have several different problems. The rules for assigning headings for treaties and works on treaties are slightly different, and the AACR2 treatment of some treaties is obtuse, to say the least (i.e. "Canada. Treaties, etc. 1992 Oct. 7" rather than NAFTA or North American Free Trade Agreement). However for most multilateral treaties, AACR2 enters them under user-friendly names, and the heading can be especially useful since international agreements are often on subjects that don't correspond nicely to the country-specific subject headings of LCSH.
It is also interesting to note that the rule governing assignment for the fixed field (in the 008) for contents limits the use of the "l" for legislation to statutes (adopted by legislatures) and regulations (adopted by agencies, though in some countries there is little substance to the distinction), but excluding constitutions and treaties. This ignores the fact the constitutions and treaties are types of legislation and have the same force (at least) as acts of the legislature or decrees of the chief executive. Of course, the fixed fields in MARC21 are from different sources than either LCSH or AACR2, so one wouldn't expect them to be neatly coordinated—but that belongs in another column.