The single subject rule is provided for by 41 state constitutions in the United States of America. Its purpose is to avoid complexity in laws, to avoid any hidden consequences that legislators or voters may miss when reading the proposed law. This in turn prevents politicians from sneaking unpopular legislation through, piggybacked on legislation which is more likely to pass.
For example, the constitution of Minnesota, Article IV, Section 17, requires that "No law shall embrace more than one subject, which shall be expressed in its title.". Conversely, neither the U.S. Congress nor the U.S. Constitution has such a rule so riders which are completely unrelated to the main bill are commonplace. These amendments are often put into bills at the last minute, so that any representative who may read the legislation before actually voting on it will not have a chance to catch it. An effort is underway, however, to amend the U.S. Constitution to add a single subject rule applicable to the U.S. Congress by an organization named Single Subject Amendment, which is a Super PAC that registered with the Federal Election Commission in March 2013.
It has been charged that single-subject rules have been misused as a political or judicial measure to slow or nullify ballots or laws. An example of accusation of misuse of this law occurred in Colorado when a former governor made statement against a single-subject ruling.  The rule can also result in overly narrow questions, that result in no substantial effects. 
- In July 2006, the Georgia Supreme Court ruled that a November 2004 amendment to the constitution of Georgia against same-sex marriage would be allowed to stand, despite also banning recognition of same-sex marriages done in other states, and banning civil unions. Additionally, voters in the referendum were told of only the same-sex marriage question, while the ballot failed to mention the other two issues, preventing voters from giving fully informed consent. A judge had previously ruled that voters had the right to decide the issue of civil unions separately, thus putting the two issues as one violated Georgia's single-subject rule.
In Swiss law, the "principle of the unity of the subject matter" (German: Grundsatz der Einheit der Materie, French: principe de l'unité de la matière) applies to ballot initiatives and to parliamentary legislation that is subject to a referendum. It has been derived by the Swiss Federal Supreme Court from the provision in article 34, section 2 of the Swiss Federal Constitution which guarantees "the freedom of the citizen to form an opinion and to give genuine expression to his or her will" in the exercise of political rights. The Court has outlined the principle as follows:
- "The principle of the unity of the subject matter requires that the subject of a referendum may, in principle, have only one topic area as its subject, that is, that two or several substantive questions or subject matters may not be joined into one referendum proposition in such a way that the voters face a dilemma and do not have a free choice between the several parts. If an item of legislation addresses several substantive questions or subject matters, the unity of the subject matter is only preserved if the several parts have a material intrinsic connection with each other, are materially related to each other and are aimed at the same goal; this material connection may not be merely artificial, subjective or political in nature. (...) Because the concept of the unity of the subject matter is a relative one, and because the weight given to the several parts of a legislative proposition and their relationship to each other is principally a political question, the authorities enjoy wide discretion in the shaping of referendum propositions."
- "Court bars immigration vote". The Denver Post. June 13, 2006. Archived from the original on June 16, 2006. Retrieved August 16, 2011.
- "Single Subject Rules". National Conference of State Legislatures. May 8, 2009. Retrieved August 16, 2011.
- Swiss Federal Supreme Court, judgment 1P.223/2006 of 12 September 2006, section 2; author's translation of the German original.