The Persian alphabet (الفبای فارسی alefbā-ye fārsi) is a writing system based on the Arabic script. Originally used exclusively for the Arabic language, the Arabic alphabet was adapted to the Persian language, adding four letters: پ [p], چ [t͡ʃ], ژ [ʒ], and گ [ɡ]. Many languages that use the Arabic script add other letters. Besides the Persian alphabet itself, the Arabic script has been applied to the Urdu alphabet, Sindhi alphabet, Saraiki alphabet, Kurdish alphabet, Lurish (Luri), Ottoman Turkish alphabet, Balochi alphabet, Punjabi Shahmukhi script, Kashmiri, Tatar, Azeri, and several others.
In order to represent non-Arabic sounds, new letters were created by adding dots, lines, and other shapes to existing letters. For example, the retroflex sounds of Urdu are represented orthographically by adding a small ط above their non-retroflex counterparts: د [d̪] and ڈ [ɖ]. The voiceless retroflex fricative [ʂ] of Pashto is represented in writing by adding a dot above and below the س [s] letter, resulting in ښ. The close back rounded vowel [u] of Kurdish is written by writing two ﻭ [u], resulting in ﻭﻭ.
The Arabic script is abjad and is exclusively written cursively. That is, the majority of letters in a word connect to each other. This is also implemented on computers. Whenever the Arabic script is typed, the computer connects the letters to each other. Unconnected letters are not widely accepted. In Arabic, as in Arabic, words are written from right to left while numbers are written from left to right.
A characteristic feature of this script, possibly tracing back to Ancient Egyptian hieroglyphs, is that vowels are underrepresented. For example, in Classical Arabic, of the six vowels, the three short ones are normally entirely omitted (although certain diacritics are added to indicate them in special circumstances, notably in the Qur'an), while the three long ones are represented ambiguously by certain consonants. Only Kashmiri, Uyghur, Kyrgyz (in China), Kazakh (in China), Kurdish and (formerly) Bosnian, of the many languages using adaptations of this script, regularly indicate all vowels.