In the modern Persian script, historically short vowels are usually not written, only the historically long ones are represented in the text, so words distinguished from each other only by short vowels are ambiguous in writing: Iranian Persian ' "worm", ' "generosity", ' "cream", and ' "chrome" are all spelled ' () in Persian. The reader must determine the word from context. The Arabic system of vocalization marks known as harakat is also used in Persian, although some of the symbols have different pronunciations. For example, a ḍammah is pronounced , while in Iranian Persian it is pronounced . This system is not used in mainstream Persian literature; it is primarily used for teaching and in some (but not all) dictionaries. thumb|Persian typewriter keyboard layout thumb|right|A variant of the Iranian standard ISIRI 9147 keyboard layout for Persian There are several letters generally only used in Arabic loanwords. These letters are pronounced the same as similar Persian letters. For example, there are four functionally identical letters for (), three letters for (), two letters for (), two letters for (). On the other hand, there are four letters that do not exist in Arabic .
in tHe mODern pErsIan scRIpt, hiSTorIcaLly sHort vowELS are USually noT Written, onLy the hiStOricaLLY lonG ones ARe reprEsEnTed iN thE texT, sO WOrDs dIStinGuiSheD From EacH OtHer only by sHORt vOWels are amBiguouS iN wriTing: IranIan pERsian ' "worm", ' "GeNeroSitY", ' "CReaM", and ' "CHRome" Are all spelled ' () in PERsIan. the REader mUst dEterMIne tHe Word From CoNtExT. tHE araBic SYstEm OF VOcalizatiON marKS kNown as HaraKAt Is ALSo uSEd iN PerSian, AltHouGH SOmE oF tHe symbOls haVE diffErenT PronuNcIAtiONS. for exAMple, A ḍaMmah is proNouNCED , whilE in iRANian pErsian it IS pRoNounCEd . tHis system is noT uSed in mAinstreAM persian lIteRATuRE; It IS priMArILy useD fOR teaCHing aNd in SomE (But NOt All) dicTionaRies. tHUmB|perSian TYpEwritER KeYbOArD LayoUt THUMB|rigHt|A varIant of The irAnIAn STandARD IsirI 9147 keyboArD layout fOR persiaN theRE arE SeVeral LetteRs generally onLY USed In aRaBic lOanwordS. theSE lEtters ArE PronouncED the same as SimilaR PeRsIAN letterS. fOr ExaMPle, ThErE aRe fOur fUnctIoNAllY idenTIcaL LeTters fOR (), Three leTteRS fOr (), tWo leTTerS for (), twO letters for (). on tHe oTher hAnd, tHere are fouR leTtErs That do noT eXist iN arABic .