The synonym planned language is sometimes used when referring to international auxiliary languages, and by those who may object to the more common term "artificial". Some speakers of Esperanto avoid the term "artificial language" because they deny that there is something "unnatural" in communicating in this language. However, outside the Esperanto community the term language planning refers to prescriptive measures taken regarding a natural language. In this regard, even "natural languages" may be submitted to a certain amount of artificiality, and in the case of regularized grammars, the line is difficult to draw.
Constructed languages are often divided into a priori languages, in which much of the grammar and vocabulary is created from scratch (using the author's imagination or automatic computational means), and a posteriori languages, where the grammar and vocabulary are derived from one or more natural languages.
Fictional and experimental languages can also be naturalistic, in the sense that they are meant to sound natural and, if derived a posteriori, they try to follow natural rules of phonological, lexical and grammatical change. Since these languages are not usually intended for easy learning or communication, a naturalistic fictional language tends to be more difficult and complex, not less (because it tries to mimic common behaviours of natural languages such as irregular verbs and nouns, complicated phonological rules, etc.).
In light of the above, most constructed languages can broadly be divided as follows:
The boundaries between these categories, however, are by no means clear. For example, there are fictional auxiliary languages, and with some constructed languages, it is hard to decide whether they are "artistic" or "engineered".
A constructed language can have native speakers if children learn it at an early age from parents who have learned the language. Esperanto has a considerable number of native speakers, variously estimated to be between 200 and 2000. A member of the Klingon Language Institute, d'Armond Speers, attempted to raise his son as a native (bilingual with English) Klingon speaker. Evan Robertson, the creator of Mosro, successfully taught the language to his four youngest children. However, as soon as a constructed language does have a number of native speakers, it begins to evolve, and thereby loses its constructed status over time. For example Modern Hebrew was modelled on Biblical Hebrew rather than engineered from scratch, and has undergone considerable changes since the state of Israel was founded in 1948.
Proponents of particular constructed languages often have many reasons for using them. Among these, the famous but disputed Sapir-Whorf hypothesis is often cited; this claims that the language one speaks influences the way in which one thinks. Thus, a "better" language should allow the speaker to reach some elevated level of intelligence, or to encompass more diverse points of view.
Grammatical speculation is documented from Classical Antiquity, with Plato's Cratylus. However the suggested mechanisms of grammar were designed to explain existing languages (Latin, Greek, Sanskrit), and not aiming at constructing new grammars. Roughly contemporary to Plato, in his descriptive grammar of Sanskrit, Pāṇini constructed a meta-linguistic formalism, so that the text of his grammar may be considered a mixture of natural and constructed language.
The earliest non-natural languages were not so much considered "constructed" as "super-natural" or mystical. The Lingua Ignota, recorded in the 12th century by St Hildegard of Bingen is an example of this, apparently it is a form of private mystical cant (see also language of angels). Kabbalistic grammatical speculation was directed at recovering the original language spoken by Adam and Eve in Paradise, lost in the confusion of tongues. The first "Christian" project for an ideal language is outlined in Dante Alighieri's De vulgari eloquentia, where he searches for the ideal Italian vernacular suited for literature. Raymond Lull's Ars magna was a project of a perfect language with which the infidels could be convinced of the truth of the "Christian" faith. It was basically an application of combinatorics on a given set of concepts. During the Renaissance, Lullian and Kabbalistic ideas were carried ad absurdum in a magical context, resulting in cryptographic applications. The Voynich manuscript may be an example of this. Renaissance interest in Ancient Egypt, notably the discovery of the Hieroglyphica of Horapollo, and first encounters with the Chinese script directed efforts towards a perfect language of written characters. Johannes Trithemius, in his works Steganographia and Polygraphia, attempted to show how all languages can be reduced to one. In the 17th century, interest in magical languages was continued by the Rosicrucians and Alchemists (like John Dee). Jakob Boehme in 1623 spoke of a "natural language" (Natursprache) of the senses.
Musical languages from the Renaissance were tied up with mysticism, magic and alchemy, sometimes also referred to as the language of the birds. The Solresol project of 1817 re-invented the concept in a more pragmatic context.
The 17th century also saw the rise of projects for "philosophical" or "a priori" languages. Pioneered by Francis Lodwick's A Common Writing (1647) and The Groundwork or Foundation laid (or So Intended) for the Framing of a New Perfect Language and a Universal Common Writing (1652), George Dalgarno (Ars signorum, 1661) and John Wilkins (Essay towards a Real Character, and a Philosophical Language, 1668) produced systems of hierarchical classification that were intended to result in both spoken and written expression. Gottfried Leibniz with lingua generalis in 1678 pursued a similar end, aiming at a lexicon of characters upon which the user might perform calculations that would yield true propositions automatically, as a side-effect developing binary calculus. These projects were not only occupied with reducing or modelling grammar, but also with the arrangement of all human knowledge into "characters" or hierarchies, an idea that with the Enlightenment would ultimately lead to the Encyclopédie.
Leibniz and the encyclopedists realized that it is impossible to organize human knowledge unequivocally in a tree diagram, and consequently to construct an a priori language based on such a classification of concepts. Under the entry Charactère, D'Alambert critically reviewed the projects of philosophical languages of the preceding century. From the Encyclopédie, projects for a priori languages moved more and more to the lunatic fringe. Individual authors, typically unaware of the history of the idea, continued to propose taxonomic philosophical languages until the early 20th century (e.g. Ro), but most recent engineered languages have had more modest goals; some are limited to a specific field, like mathematical formalism or calculus (e.g. Lincos and programming languages), others are designed for eliminating syntactical ambiguity (e.g., Loglan and Lojban) or maximizing conciseness (e.g., Ithkuil).
Already in the Encyclopédie attention began to focus on a posteriori auxiliary languages. Joachim Faiguet in the article on Langue already wrote a short proposition of a "laconic" or regularized grammar of French. During the 19th century, a bewildering variety of such International Auxiliary Languages (IALs) were proposed, so that Louis Couturat and Leopold Leau in Historire de la langue universelle (1903) could review 38 projects. The first of these that made any international impact was Volapük, proposed in 1879 by Johann Martin Schleyer, and within a decade, 283 Volapükist clubs were counted all over the globe. However, this language by its very success lost its unity, and within a few years, fell into obscurity, making way for Esperanto, proposed in 1887 by Ludwik Lejzer Zamenhof, the most successful IAL to date. Loglan (1955) and its descendants constitute a pragmatic return to the aims of the a priori languages, tempered by the requirement of usability of an auxiliary language.
Artistic languages, constructed for literary enjoyment or aesthetic reasons without any claim of usefulness, begin to appear in Early Modern literature (in Pantagruel, and in Utopian contexts), but they only seem to gain notability as serious projects from the 20th century. A Princess of Mars by Edgar Rice Burroughs was possibly the first fiction of the 20th century to feature a constructed language. J. R. R. Tolkien was the first to develop a family of related fictional languages and was the first academic to publicly discuss artistic languages, admitting to A Secret Vice of his in 1930 at an Esperanto congress. (George Orwell's Newspeak should be considered a parody of an IAL rather than an artistic language proper.)
By the turn of the 21st century, it had become common for science-fiction and fantasy works set in other worlds to feature constructed languages, and constructed languages are a regular part of movies of the genre, including Star Wars, Star Trek, and Atlantis: The Lost Empire.