[4] The complete katakana script consists of 48 characters, not counting functional and diacritic marks: These are conceived as a 5×10 grid (gojūon, 五十音, literally "fifty sounds"), as shown in the adjacent table, read ア (a), イ (i), ウ (u), エ (e), オ (o), カ (ka), キ (ki), ク (ku), ケ (ke), コ (ko) and so on.

In vertical text contexts, which used to be the default case, the grid is usually presented as 10 columns by 5 rows, with vowels on the right hand side and ア (a) on top.

Three of the syllabograms to be expected, yi, ye and wu, may have been used idiosyncratically with varying glyphs, but never became conventional in any language and are not present at all in modern Japanese.

The script includes two diacritic marks placed at the upper right of the base character that change the initial sound of a syllabogram.

A double dot, called dakuten, indicates a primary alteration; most often it voices the consonant: k→g, s→z, t→d and h→b; for example, カ (ka) becomes ガ (ga).

The layout of the gojūon table promotes a systematic view of kana syllabograms as being always pronounced with the same single consonant followed by a vowel, but this is not exactly the case (and never has been).

Both approaches conceal the fact, though, that many consonant-based katakana signs, especially those canonically ending in u, can be used in coda position, too, where the vowel is unvoiced and therefore barely perceptible.

A character called a sokuon, which is visually identical to a small tsu ッ, indicates that the following consonant is geminated (doubled).

In Japanese this is an important distinction in pronunciation; for example, compare サカ saka "hill" with サッカ sakka "author".

Geminated consonants are common in transliterations of foreign loanwords; for example, English "bed" is represented as ベッド (beddo).

The sokuon may also be used to approximate a non-native sound: Bach is written バッハ (Bahha); Mach as マッハ (Mahha).

Small versions of the five vowel kana are sometimes used to represent trailing off sounds (ハァ haa, ネェ nee), but in katakana they are more often used in yōon-like extended digraphs designed to represent phonemes not present in Japanese; examples include チェ (che) in チェンジ chenji ("change"), ファ (fa) in ファミリー famirī ("family") and ウィ (wi) and ディ (di) in ウィキペディア Wikipedia; see below for the full list.

Technical and scientific terms, such as the names of animal and plant species and minerals, are also commonly written in katakana.

[7] Homo sapiens, as a species, is written ヒト (hito), rather than its kanji 人. Katakana are often (but not always) used for transcription of Japanese company names.

Katakana are commonly used on signs, advertisements, and hoardings (i.e., billboards), for example, ココ (koko, "here"), ゴミ (gomi, "trash"), or メガネ (megane, "glasses").

Words the writer wishes to emphasize in a sentence are also sometimes written in katakana, mirroring the usage of italics in European languages.

There are rare instances where the opposite has occurred, with kanji forms created from words originally written in katakana.

For example, in a manga, the speech of a foreign character or a robot may be represented by コンニチワ konnichiwa ("hello") instead of the more typical hiragana こんにちは.

This was particularly common among women in the Meiji and Taishō periods, when many poor, illiterate parents were unwilling to pay a scholar to give their daughters names in kanji.

Some examples include マンガ ("manga"), アイツ aitsu ("that guy or girl; he/him; she/her"), バカ baka ("fool"), etc.

Some instructors teaching Japanese as a foreign language "introduce katakana after the students have learned to read and write sentences in hiragana without difficulty and know the rules.

Characters shi シ, tsu ツ, so ソ, and n ン look very similar in print except for the slant and stroke shape.

nu- mu- ru- Katakana was developed in the 9th century (during the early Heian period) by Buddhist monks in Nara in order to transliterate texts and works of arts from India, by taking parts of man'yōgana characters as a form of shorthand, hence this kana is so-called kata (片, "partial, fragmented").

[19] Katakana is also heavily influenced by Sanskrit due to the original creators having travelled and worked with Indian Buddhists based in East Asia during the era.

Katakana's choices of man'yōgana segments had stabilized early on and established – with few exceptions – an unambiguous phonemic orthography (one symbol per sound) long before the 1900 script regularization.

Although their display form is not specified in the standard, in practice they were designed to fit into the same rectangle of pixels as Roman letters to enable easy implementation on the computer equipment of the day.

When originally devised, the half-width katakana were represented by a single byte each, as in JIS X 0201, again in line with the capabilities of contemporary computer technology.

Several popular Japanese encodings such as EUC-JP, Unicode and Shift JIS have half-width katakana code as well as full-width.

Extensions to Katakana for phonetic transcription of Ainu and other languages were added to the Unicode standard in March 2002 with the release of version 3.2.

The CJK Compatibility block contains in the range U+3330-3357 square versions of katakana words, usually measure units or abbreviations of loanwords: