Why can Tajik speakers generally understand Persian?

Tajik speakers are generally able to understand Persian because Tajik and Persian are very closely related languages within the southwest group of Iranian languages.12

Because of this close linguistic kinship, the two languages are considered mutually intelligible. While Tajik is sometimes described as being more archaic in certain respects than modern Persian, the two share a common historical and linguistic foundation. This relationship allows speakers of one to communicate effectively with speakers of the other, despite geographical separation and differences in writing systems—such as Tajik’s use of the Cyrillic alphabet compared to the Arabic characters typically used for Persian.3214