[tag resources tasdid-1-43c]Tašdid,[/tag] تشدید literally, ‘gemination,’ is sometimes called “doubling.” In Persian, when in a single morpheme (word) a segment of adjacent identical consonants occurs (that is, a consonantal sound followed by a similar consonantal sound in a following syllable, creating a cluster; as in, [bb], [cc], [dd], etc.), and the segment is followed by a vowel (V) (that is, [bbV], [ccV], [ddV], etc.), a gemination occurs. In the Perso-Arabic writing system, the two sounds are contracted—that is to say, only one is written—and a [ ّ ] sign, called tašdid (šadda in Arabic, which literally means ‘to intensify’), is placed over the character as a (sort of) diacritic.

Occasionally in both Arabic and Persian tašdid may occur terminally; as in, [tag resources tasdid-1-15a]همّ[/tag] hamm, Arabic for ‘care, solicitude,’ [tag resources tasdid-1-16a]حیّ[/tag] hayy, Arab for ‘alive,’ [tag resources tasdid-1-16b]فرّ[/tag] farr, Persian ‘fortune, glory, splendor.’ While this occurrence is normal in Arabic, it is very rare in Persian; for, as was explained, under normal circumstances, the segment must be followed by a vowel (which obviously is impossible in a final position); as in, [tag resources tasdid-1-19a]بچّه[/tag] bacce ‘child,’ [tag resources tasdid-1-19b]فرّخ[/tag] farrox ‘fortunate, blessed, happy.’ Therefore, cases such as farr have historical and etymological reasons for a terminal doubling. For instance, farr is a later development of xwarrah (~ xwarna-)—already with a medial geminated [-rr-], etc.

Words may have a combination of [tag resources ps Madd-1-18d]مدّ[/tag] ,[tag resources ps Madd-1-18c]تشدید[/tag] ,[tag resources ps Madd-1-18b]همزه[/tag] ,[tag resources ps Madd-1-18a]تنوین[/tag] ; as in: [tag resources ps Madd-1-18e]اوّلاً[/tag], in which the و is mošaddad (‘having tašdid’; i.e., ‘geminated’), and the terminal alef has tanvin; [tag resources Hamze-2-40a]تأثّر[/tag], in which there is a combination of hamze and tašdid, etc.

Also, bear in mind that the و in Persian numbers دوم dovom and سوم sevom must not be geminated. Therefore, the common pronunciation of these words as [tag resources Hamze-3-04a]دوّم[/tag] dovvom and [tag resources Hamze-3-04b]سوّم[/tag] sevvom is only on analogy with the geminated و in the Arabic اوّل awwal ‘first’ and اوّلاً awwalan ‘firstly’ (Persian avval, avvalan) (that is to say, dovom and sevom, without a geminated [v] are the correct pronunciations).