Which Byzantine church in Istanbul is now known as Kalenderhane Mosque?

Istanbul’s Kalenderhane Mosque is identified with the Byzantine Church of the Theotokos Kyriotissa.

The church stands near the Aqueduct of Valens in Constantinople. Archaeological and architectural evidence indicates that the surviving building incorporates earlier structures and was developed in the middle or late Byzantine period.

Its original dedication is commonly given as the Theotokos Kyriotissa, meaning the Virgin Mary as Lady or Sovereign. The church had a cross-in-square-like organization and was decorated with marble revetment and mosaics.

After the Ottoman conquest of Constantinople, the building was converted into a mosque and became known as Kalenderhane Mosque. Scholars have debated details of its earlier history, including the exact sequence of construction phases, but the identification with the Kyriotissa church is the standard association.