İstanbul was the common name for the city in normal speech in Turkish even since before the conquest of 1453, but in official use by the Ottoman authorities, other names such as Kostantiniyye were preferred in certain contexts. Thus, Kostantiniyye was used on coinage up to the late 17th and then again in the 19th century. The Ottoman chancelery and courts used Kostantiniyye as part of intricate formulae in expressing the place of origin of formal documents, such as be-Makam-ı Darü's-Saltanat-ı Kostantiniyyetü'l-Mahrusâtü'l-Mahmiyye In 19th century Turkish bookprinting it was also used in the impressum of books, in contrast to the foreign use ofConstantinople. At the same time, however, İstanbul too was part of the official language, for instance in the titles of the highest Ottoman military commander(İstanbul ağası) and the highest civil magistrate (İstanbul efendisi) of the city. İstanbul and several other variant forms of the same name were also widely used in Ottoman literature and poetry.
After the creation of the Republic of Turkey in 1923, the various alternative names besides İstanbul became obsolete in the Turkish language. With the Turkish Postal Service Law of March 28, 1930, the Turkish authorities officially requested foreigners to cease referring to the city with their traditional non-Turkish names (such as Constantinople, Tsarigrad, etc.) and to adopt Istanbul as the sole name also in their own languages. Letters or packages sent to "Constantinople" instead of "Istanbul" were no longer delivered by Turkey's PTT, which contributed to the eventual worldwide adoption of the new name.
In English the name is usually written "Istanbul". In modern Turkish the name is written "İstanbul" because in the Turkish alphabet dotted i (capital İ) is a different letter from dotless ı (capital I).