The Nishan Sahib is the flag flown outside every gurdwara, a triangular pennant on a tall pole wrapped in the same cloth, carrying the khanda emblem.
It is generally saffron, basanti, in mainstream practice, and deep blue, surmai, in the Nihang tradition. The colour is one of the visible markers between them.
The cloth is changed annually at Baisakhi in a ceremony in which the pole is lowered, washed with milk and water, rewrapped and raised.
It marks the gurdwara from a distance and it historically marked a place where a traveller could get food and shelter, which is a practical function in flat country.
The khanda emblem itself, a double-edged sword between two kirpans within a chakkar, is a twentieth-century standardisation of older devices and its modern form dates from the Singh Sabha period rather than from the Gurus.
The pole is wrapped in cloth of the same colour and topped with a khanda. Changing the cloth is done by hand with the pole lowered and washed, and the crowd takes pieces of the old cloth away.
The right to fly it was not always free. Under the Mughal and Afghan administrations a Sikh flag over a building marked it for destruction, and the flags went up permanently only once the misls held the country.
The saffron and blue distinction marks practice rather than doctrine. Nihang deras fly blue, most gurdwaras fly saffron, and a few fly white.