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 (“tashdid”) over the geminated consonant. Tashdid is a mark an like other marks used in vowel marking and diphthong marking, it is not normally written. 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 (نجار) the two “j's” 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 khayyât khay-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
khatt khat
khatti khat-ti

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