Until 1990, Yemen was divided into North Yemen and South Yemen, each with its own flag.

The flag of Yemen in the northern part
After the declaration of independence of the northern part of Yemen and its separation from the Ottoman Empire, the Yemeni Mutawakkilite Kingdom was established with a flag on a red background with 5 white stars and a saber. This was between 1927 and 1962, and starting from that date, a revolution broke out in the northern part with the support of Egypt, leading to the creation of the Yemeni Arab Republic and a new flag: three horizontal lines of red, white and black and a green star in the middle of a white stripe.
The flag of Yemen in the southern part
In 1967, nationalist forces similar to those in the north succeeded in the southern part of Yemen and were able to overthrow British rule and proclaim a republic. Southern Yemen existed until it officially became the People's Democratic Republic of Yemen on November 30, 1967, and its flag also had the same three colors: red, white and black, but with a blue triangle on the left side of the flag, similar to the flag of the Czech Republic, and a red star in the middle of the triangle.
Yemen's flag after unification
In 1990, North and South Yemen were united under one government and the name of the Republic of Yemen. Since the difference between the northern and southern flags was only in symbolism, the unified flag took on the appearance of a red, white, and black tricolor with additional elements such as the star and triangle removed. This flag has remained unchanged to this day.