Gemination is the doubling of a consonant sound. A geminated consonant is articulated for a longer period of time than that of a single consonant. In English, gemination does not occur within words but for example in “calm man”, the consonant “m” is geminated.

In Latin script, gemination is represented with a doubled letter but in Persian script, gemination is marked by placing the diacritic تشدید (“tašdîd”) over the geminated consonant. Tashdid is a mark and therefore, it is not normally written just like the other marks used in vowel marking and diphthong marking. You have to memorize words having a geminated consonant. When Persian is written in Latin alphabet, gemination can be denoted with double consonants as in Italian. For example, in “najjâr” (نجّار) the “j” letters fall into separate syllables and the word is pronounced as “naj-jâr”:

Persian Script Latin Script Syllables
امّا ammâ am-mâ
پلّه pelle pel-le
نجّار najjâr naj-jâr
جلّاد jallâd jal-lâd
خیّاط xayyât xay-yât

When gemination occurs at the end of a word, geminated consonants do not fall into separate syllables and therefore, they are pronounced as one consonant. The gemination of such words is revealed when they take a suffix and their geminated consonant is no longer at final position.

Persian Script Latin Script Syllables
xatt xat
xatti xat-ti

Here are a few words that have geminated consonants. They are normally written without gemination mark (Tashdid):