Analogy in law is a method of resolving issues on which there is no previous authority by using argument from analogy. Analogy in general involves an inference drawn from one particular situation to another based on similarity, but legal analogy is distinguished by the need to use a legally relevant basis for drawing an analogy between two situations. It may be applied to various forms of legal authority, including statutory law and case law.
In the civil law and common law traditions, the basis of legal relevance that allows drawing a legally valid analogy is described by different terms depending on the source of law involved: ratio decidendi for precedent, ratio legis for statutory law, and ratio iuris for unwritten legal principles. The use of analogy in both traditions is broadly described by the traditional maxim Ubi eadem est ratio, ibi idem ius (where the reason is the same, the law is the same).
Although all legal systems use analogy in some fashion, different jurisdictions and legal traditions apply or limit analogy in many different ways. The civil law and common law traditions differ most prominently in the subject matter to which analogy is typically applied: in civil law courts, analogy is most typically employed to fill in gaps in a statute, while in common law courts it is most commonly used to apply and extend precedent. In addition, these legal systems have developed elaborate typologies of analogy, although these are often disputed.
The analogical extension of criminal penalties ("punishment by analogy") and tax liability is prohibited in many modern jurisdictions, under the various legal principles that safeguard legal certainty. Historically, however, punishment by analogy has been part of many legal systems, including those of imperial China, the early USSR, and the People's Republic of China prior to 1998. A few countries have retained legal provisions that at least nominally allow for punishment by analogy.