Originally a Semitic writing system, the modern day Perso-Arabic script has retained two essential features:

• It is written from right to left (compare it, for example, with Hebrew, another Semitic script). In Arabic this writing system has 28 characters; in Persian, 32 (as there are no p, ch, zh, and g, in Arabic). With the addition of the hamze (the glottal stop, which is exclusively used in the transcription of the loan-words from Arabic) there are 33 characters in Persian.
• It is a consonantal writing system; that is, a system in which the short vowels are not included. Instead, diacritics may be used above or below a character to apply a short vowel to it. However, these diacritics are hardly ever used, with the exception of the holy Qurān, which is supposed to be read properly (i.e., with words pronounced correctly), otherwise it would lose its magical and spiritual powers.

In addition, Persian shares a particular feature with many other Indo-European languages: the historical orthography—that is, spelling words in their original and historical pronounced/written form, regardless of the later developments. Compare that with the English spelling of words such as ‘daughter,’ instead of the hypothetical spelling of the word as dawter for instance, because at one time, this was how the word was pronounced (compare with the German tochter and Persian doxtar). Also compare English ‘light’ and German ‘licht,’ which reflect the presence of a voiceless velar fricative of some sort at some stage of the development of the word. Such Persian words as خواهر xwāhar (pronounce xāhar) reflect an earlier /w/ between the /x/ and /ā/. This /w/ is still pronounced in some northern Iranian dialects, such as the Dari of Afghanistan, etc.

Finally, the Arabic loanwords are spelled as they are written in Arabic, but they are pronounced according to the Persian phonetic system.