# Apertium

Apertium is a free/open-source rule-based machine translation platform. It is free software and released under the terms of the GNU General Public License.

## Overview

Apertium is a transfer-based machine translation system, which uses finite state transducers for all of its lexical transformations, and Constraint Grammar taggers as well as hidden Markov models or Perceptrons for part-of-speech tagging / word category disambiguation. A structural transfer component is responsible for word movement and agreement; most Apertium language pairs up until now have used "chunking" or shallow transfer rules, though newer pairs use (possibly recursive) rules defined in a Context-free grammar.
Many existing machine translation systems available at present are commercial or use proprietary technologies, which makes them very hard to adapt to new usages. Apertium code and data is free software and uses a language-independent specification, to allow for the ease of contributing to Apertium, more efficient development, and enhancing the project's overall growth.
At present (December 2020), Apertium has released 51 stable language pairs, delivering fast translation with reasonably intelligible results (errors are easily corrected). Being an open-source project, Apertium provides tools for potential developers to build their own language pair and contribute to the project.

## History

Apertium originated as one of the machine translation engines in the project OpenTrad, which was funded by the Spanish government, and developed by the Transducens research group at the Universitat d'Alacant. It was originally designed to translate between closely related languages, although it has recently been expanded to treat more divergent language pairs. To create a new machine translation system, one just has to develop linguistic data (dictionaries, rules) in well-specified XML formats.
Language data developed for it (in collaboration with the Universidade de Vigo, the Universitat Politècnica de Catalunya and the Universitat Pompeu Fabra) currently support (in stable version) the Arabic, Aragonese, Asturian, Basque, Belarusian, Breton, Bulgarian, Catalan, Crimean Tatar, Danish, English, Esperanto, French, Galician, Hindi, Icelandic, Indonesian, Italian, Kazakh, Macedonian, Malaysian, Maltese, Northern Sami, Norwegian (Bokmål and Nynorsk), Occitan, Polish, Portuguese, Romanian, Russian, Sardinian, Serbo-Croatian, Silesian, Slovene, Spanish, Swedish, Tatar, Ukrainian, Urdu, and Welsh languages. A full list is available below. Several companies are also involved in the development of Apertium, including Prompsit Language Engineering, Imaxin Software and Eleka Ingeniaritza Linguistikoa.
The project has taken part in the 2009, 2010, 2011, 2012, 2013 and 2014 editions of Google Summer of Code and the 2010, 2011, 2012, 2013, 2014, 2015, 2016 and 2017 editions of Google Code-In.

## Translation methodology

This is an overall, step-by-step view how Apertium works.
The diagram displays the steps that Apertium takes to translate a source-language text (the text we want to translate) into a target-language text (the translated text).
* Source language text is passed into Apertium for translation.
* The deformatter removes formatting markup (HTML, RTF, etc.) that should be kept in place but not translated.
* The morphological analyser segments the text (expanding elisions, marking set phrases, etc.), and looks up segments in the language dictionaries, returning dictionary forms and tags for all matches. In pairs that involve agglutinative morphology, including a number of Turkic languages, a Helsinki Finite State Transducer (HFST) is used. Otherwise, an Apertium-specific finite state transducer system called lttoolbox, is used.
* The morphological disambiguator (the morphological analyser and the morphological disambiguator together form the part of speech tagger) resolves ambiguous segments (i.e., when there is more than one match) by choosing one match. Apertium uses Constraint Grammar rules (with the vislcg3 parser) for most of its language pairs.
* Retokenisation uses a finite state transducer to match sequences of lexical units and may reorder or translate tags (often used for translating idiomatic expressions into something that more approaches the target language grammar)
* Lexical transfer looks up disambiguated source-language basewords to find their target-language equivalents (i.e., mapping source language to target language). For lexical transfer, Apertium uses an XML-based dictionary format called bidix.
* Lexical selection chooses between alternative translations when the source text word has alternative meanings. Apertium uses a specific XML-based technology, apertium-lex-tools, to perform lexical selection.
* Structural transfer (i.e., it is an XML format that allows writing complex structural transfer rules) can consist of one-step chunking transfer, three-step chunking transfer or a CFG-based transfer module. The chunking modules flag grammatical differences between the source language and target language (e.g. gender or number agreement) by creating a sequence of chunks containing markers for this. They then reorder or modify chunks in order to produce a grammatical translation in the target-language. The newer CFG-based module matches input sequences into possible parse trees, selecting the best-ranking one and applying transformation rules on the tree.
* The morphological generator uses the tags to deliver the correct target language surface form. The morphological generator is a morphological transducer, just like the morphological analyser. A morphological transducer both analyses and generates forms.
* The post-generator makes any necessary orthographic changes due to the contact of words (e.g. elisions).
* The reformatter replaces formatting markup (HTML, RTF, etc.) that was removed by the deformatter in the first step.
* Apertium delivers the target-language translation.

## Supported languages

As of January 2025, the following 108 pairs and 50 languages and languages varieties are supported by Apertium.
* Afrikaans to Dutch
* Arabic to Maltese
* Aragonese to Catalan
* Aragonese to Spanish
* Arpitan (Franco-Provençal) to French
* Basque to English
* Basque to Spanish
* Belarusian to Russian
* Breton to French
* Bulgarian to Macedonian
* Catalan to Aragonese
* Catalan to English
* Catalan to Esperanto
* Catalan to French
* Catalan to Italian
* Catalan to Occitan
* Catalan to Aranese
* Catalan to Portuguese
* Catalan to Brazilian Portuguese
* Catalan to European Portuguese (traditional spelling)
* Catalan to Romanian
* Catalan to Sardinian
* Catalan to Spanish
* Crimean Tatar to Turkish
* Danish to Norwegian (Bokmål)
* Danish to Norwegian (Nynorsk)
* Danish to Swedish
* Dutch to Afrikaans
* English to Catalan
* English to Valencian
* English to Esperanto
* English to Galician
* English to Serbo-Croatian
* English to Spanish
* Esperanto to English
* French to Arpitan (Franco-Provençal)
* French to Catalan
* French to Esperanto
* French to Occitan
* French to Gascon
* French to Spanish
* Galician to English
* Galician to Portuguese
* Galician to Spanish
* Hindi to Urdu
* Icelandic to English
* Icelandic to Swedish
* Indonesian to Malay
* Italian to Catalan
* Italian to Sardinian
* Italian to Spanish
* Kazakh to Tatar
* Macedonian to Bulgarian
* Macedonian to English
* Malay to Indonesian
* Maltese to Arabic
* Northern Sámi to Norwegian (Bokmål)
* Norwegian (Bokmål) to Danish
* Norwegian (Bokmål) to Norwegian (Nynorsk)
* Norwegian (Bokmål) to East Norwegian, vi→vi
* Norwegian (Bokmål) to Swedish
* Norwegian (Nynorsk) to Danish
* Norwegian (Nynorsk) to Norwegian (Bokmål)
* Norwegian (Nynorsk) to East Norwegian, vi→vi
* Norwegian (Nynorsk) to Swedish
* East Norwegian, vi→vi to Norwegian (Nynorsk)
* Occitan to Catalan
* Occitan to French
* Occitan to Spanish
* Aranese to Catalan
* Aranese to Spanish
* Gascon to French
* Polish to Silesian
* Portuguese to Catalan
* Portuguese to Galician
* Portuguese to Spanish
* Romanian to Catalan
* Romanian to Spanish
* Russian to Belarusian
* Russian to Ukrainian
* Sardinian to Italian
* Serbo-Croatian to English
* Serbo-Croatian to Macedonian
* Serbo-Croatian to Slovenian
* Silesian to Polish
* Slovenian to Serbo-Croatian
* Spanish to Aragonese
* Spanish to Asturian
* Spanish to Catalan
* Spanish to Valencian
* Spanish to English
* Spanish to Esperanto
* Spanish to French
* Spanish to Galician
* Spanish to Italian
* Spanish to Occitan
* Spanish to Aranese
* Spanish to Portuguese
* Spanish to Brazilian Portuguese
* Swedish to Danish
* Swedish to Icelandic
* Swedish to Norwegian (Bokmål)
* Swedish to Norwegian (Nynorsk)
* Tatar to Kazakh
* Turkish to Crimean Tatar
* Ukrainian to Russian
* Urdu to Hindi
* Welsh to English