Often Islam is associated with a symbol of the crescent moon and a star. This symbolism may be related to the fact that the lunar calendar plays a significant role in Islam. Some historical sources posit that the symbol was appropriated from the Byzantines when Muslim forces defeated them in the late seventh century. In any case, the crescent and star icon does not constitute an official symbol in Islam, though it adorns many countries' flags, currency, masjids and other structures.