Wikipedia
The kokoshnik is a traditional Russian head-dress worn by women and girls to accompany the sarafan, primarily worn in the northern regions of Russia in the 16th to 19th centuries.
Historically a kokoshnik is a headdress worn by married women, though maidens wore a headdress very similar to a kokoshnik, but open in the back, named a povyazka. The word kokoshnik describes a great variety of headdresses worn throughout Russia, including the cylindrical hats of Veliky Novgorod, two-pointed nimbus kika of Vladimir, triangular kika of Kostroma, small pearl hats of Kargopol, and scarlet kokoshniks of Moscow.
While in the past kokoshnik styles varied greatly, currently a kokoshnik is generally associated with a tall, nimbus or crest shaped headdress which is tied at the back of the head with long thick ribbons in a large bow. The crest can be embroidered with pearls and goldwork or simple applique, usually using plant and flower motifs. The forehead area is frequently decorated with pearl netting. While wearing a kokoshnik the woman usually wears her hair in a plait. It resembles the French hood worn in Tudor England, but without the black veil.
Kokoshnik is a semicircular or keel-like exterior decorative element in the traditional Russian architecture, a type of corbel zakomara (that is an arch-like semicircular top of the church wall). Unlike zakomara that continues the curvature of the vault behind and carries a part of the vault's weight, kokoshnik is pure decoration and doesn't carry any weight. Kokoshnik shares its name with the traditional Russian head-dress worn by women and girls.
Kokoshniks were used in the Russian church architecture from the 16th century, and they were especially popular in the 17th century. They were placed on walls, at the basement of tented roofs or tholobates, over the window frames, or in rows above the vaults.