There are many explications for the meaning of the bindi. It's name comes from the Sanskrit word bindu (drop, point) and it's meant to be an auspicious symbol.
Depending on the region men as well as women paint their foreheads. Especially men show their religious preferences this way. For example 3 white stripes show a follower of Shiva. The application of kumkum powder is also part of religious rituals. It's not aesthetics that counts here but the colour is put on the 3rd eye chakra to lead the attention on divine power and spiritual enlightenment.
For example a woman would end her morning prayers by applying first a dab of sandalwood paste in the middle of her forehead, then on top of this put a circle of vermilion red and on top a few grains of rice.
In earlier days, Kshatriya queens used to put a tilak on their husband's forehead to bring him luck in the battlefield or to greet him when coming home. You can still find this tradition as a way of welcoming guests.
The special bindi of the women developed through the wedding ritual of the Hindus. It was painted as a sign of the wife together with red powder on the top of her head. Still today it is a part of the bride's make-up. The red colour is meant to bring prosperity to her home, the dot makes her the keeper of welfare. And of course it shows the other people that this woman is no longer available... In northern India the bindi is still mostly a sign for a married woman, in the South it is worn by everybody, even small children.
Of course a bindi is also an important accessory for Indian dance - we don't even practice without one. Traditionally it is the red, round one. The dancers of old were not allowed to have husbands but they were kind of married to a god.

In the 18th century rich Indian women could afford glamorous Bindis made of gold, diamonds and pearls. With these sophisticated styles the meaning also changed from religious to fashionable.

The bindi of wifes still is mostly a red dot and conservative women still use kumkum for it. But the self-adhesive "fancy bindis" are becoming more popular and nowadays they are matched to the colours of clothing. Nowadays bindis are produced in all sizes and of different materials. There are simple ones and those with little mirrors, shells and peals or the more elaborate ones with glittering stones. The bindi on the forehead is supposed to enchant the lover... Even poems have been written through the times about a ladie's beautiful bindis.

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