There are many consistent or standardized romanization systems. They can be classified by their characteristics. A particular system’s characteristics may make it better-suited for various, sometimes contradictory applications, including document retrieval, linguistic analysis, easy readability, faithful representation of pronunciation.
If the romanization attempts to transliterate the original script, the guiding principle is a one-to-one mapping of characters in the source language into the target script, with less emphasis on how the result sounds when pronounced according to the reader's language. For example, the Nihon-shiki romanization of Japanese allows the informed reader to reconstruct the original Japanese kana syllables with 100% accuracy, but requires additional knowledge for correct pronunciation.
Most romanizations are intended to enable the casual reader who is unfamiliar with the original script to pronounce the source language reasonably accurately. Such romanizations follow the principle of phonemic transcription and attempt to render the significant sounds (phonemes) of the original as faithfully as possible in the target language. The popular Hepburn romanization of Japanese is an example of a transcriptive romanization designed for English speakers.
A phonetic conversion goes one step further and attempts to depict all phones in the source language, sacrificing legibility if necessary by using characters or conventions not found in the target script. In practice such a representation almost never tries to represent every possible allophone—especially those that occur naturally due to coarticulation effects—and instead limits itself to the most significant allophonic distinctions. The International Phonetic Alphabet is the most common system of phonetic transcription.
For most language pairs, building a usable romanization involves tradeoffs between the two extremes. Pure transcriptions are generally not possible, as the source language usually contains sounds and distinctions not found in the target language, but which must be shown for the romanized form to be comprehensible. Furthermore, due to diachronic and synchronic variance no written language represents any spoken language with perfect accuracy and the vocal interpretation of a script may vary by a great degree among languages. In modern times the chain of transcription is usually spoken foreign language, written foreign language, written native language, spoken (read) native language. Reducing the number of those processes, i.e. removing one or both steps of writing, usually leads to more accurate oral articulations. In general, outside a limited audience of scholars, romanizations tend to lean more towards transcription. As an example, consider the Japanese martial art 柔術: the Nihon-shiki romanization zyûzyutu may allow someone who knows Japanese to reconstruct the kana syllables じゅうじゅつ, but most native English speakers or rather readers would find it easier to guess the pronunciation from the Hepburn version, jūjutsu.
The Arabic alphabet is used to write Arabic, Persian, Urdu and Pashto as well as numerous other languages in the Muslim world, particularly African and Asian languages without alphabets of their own. Romanization standards include the following: