A back-translation is a translation of a translated text back into the language of the original text, made without reference to the original text. In the context of machine translation, this is also called a round-trip translation. It is analogous to reversing a mathematical operation; but even in mathematics such a reversal frequently does not produce a value that is precisely identical with the original.
Mark Twain Comparison of a back-translation to the original text is sometimes used as a quality check on the original translation. But while useful as an approximate check, it is far from infallible. Humorously telling evidence for this was provided by Mark Twain when he issued his own back-translation of a French version of his famous short story, “The Celebrated Jumping Frog of Calaveras County”.
In cases when a historic document survives only in translation, the original having been lost, researchers sometimes undertake back-translation in an effort to reconstruct the original text. An example involves the novel The Saragossa Manuscript by the Polish aristocrat Jan Potocki (1761–1815). The Polymath Polyglot composed the book entirely in French and published fragments anonymously in 1804 and 1813–14. Portions of the original French-language manuscripts were subsequently lost; the missing fragments survived, however, in a Polish translation that was made by Edmund Chojecki in 1847 from a complete French copy, now lost. French-language versions of the complete Saragossa Manuscript have since been produced, based on extant French-language fragments and on French-language versions that have been back-translated from Chojecki’s Polish version.
Similarly, when historians suspect that a document is actually a translation from another language, back-translation into that hypothetical original language can provide supporting evidence by showing that such characteristics as idioms, puns, peculiar grammatical structures, etc., are in fact derived from the original language.
For example, the known text of the Till Eulenspiegel folk tales is in High German but contains many puns which only work if back-translated into Low German. This seems clear evidence that these tales (or at least large portions of them) were originally composed in Low German and rendered into High German by an over-metaphrastic translator.
Similarly, supporters of Aramaic primacy—i.e., of the view that the Christian New Testament or its sources were originally written in the Aramaic language—seek to prove their case by showing that difficult passages in the existing Greek text of the New Testament make much better sense if back-translated into Aramaic—that, for example, some incomprehensible references are in fact Aramaic puns which do not work in Greek.