Mutatis mutandis is a Medieval Latin phrase meaning "with things changed that should be changed" or "having changed what needs to be changed" or "once the necessary changes have been made". It remains unnaturalized in English and is therefore usually italicized in writing. It is used in many countries to acknowledge that a comparison being made requires certain obvious alterations, which are left unstated. It is not to be confused with the similar ceteris paribus, which excludes any changes other than those explicitly mentioned. Mutatis mutandis is still used in law, economics, mathematics, linguistics and philosophy. In particular, in logic, it is encountered when discussing counterfactuals, as a shorthand for all the initial and derived changes which have been previously discussed.
The phrase mutatis mutandis—now sometimes written mūtātīs mūtandīs to show vowel length—does not appear in surviving classical literature. It is Medieval Latin in origin and the Feet of fines, kept at the The National Archives (United Kingdom), contains its first use in Great Britain on January 20, 1270, at Pedes Finium, 54 Hen. III, Salop.
Both words are participles of the Latin verb mutare ('to move; to change; to exchange'). Mutatus, -a, -um is its perfect passive participle ('changed; having been changed'). Mutandus, -a, -um is its gerundive, which functions both as a future passive participle ('to be changed; going to be changed') and as a verbal adjective or noun expressing necessity ('needing to be changed; things needing to be changed'). The phrase is an ablative absolute, using the ablative case to show that the clause is grammatically independent ('absolute' literally meaning 'dissolved' or 'separated') from the rest of the sentence.
Mutatis mutandis was first borrowed into English in the 16th century, but continues to be italicized as a foreign phrase. Although many similar adverbial phrases are treated as part of the sentence, mutatis mutandis is usually set apart by commas or in some other fashion. The nearest English equivalent to an ablative absolute is the nominative absolute, so that a literal translation will either use the nominative case ("those things which need to be changed having been changed") or a preposition ("with the things needing to be changed having been changed"). More often, the idea is expressed more tersely ("with the necessary changes") or using subordinating conjunctions and a dependent clause ("once the necessary adjustments are made").
The phrase has a technical meaning in mathematics where it is sometimes used to signal that a proof can be more generally applied to other certain cases after making some, presumably obvious, changes. It serves a similar purpose to the more common phrase, "without loss of generality" (WLOG).
The legal use of the term is somewhat specialized. As glossed by Shira Scheindlin, judge for the Southern District of New York, for a 1998 case: "This Latin phrase simply means that the necessary changes in details, such as names and places, will be made but everything else will remain the same." In the wake of the Plain English movements, some countries attempted to replace their law codes' legal Latin with English equivalents.
The phrase appears in other European languages as well. A passage of Marcel Proust's Remembrance of Things Past includes "...j'ai le fils d'un de mes amis qui, mutatis mutandis, est comme vous..." ("A friend of mine has a son whose case, mutatis mutandis, is very much like yours.") The German Ministry of Justice, similar to the Plain English advocates above, now eschews its use. Their official English translation of the Civil Code now reads:
"Section 27 (Appointment of and management by the board). ...(3) The management by the board is governed by the provisions on mandate in sections 664 to 670 with the necessary modifications."