On Rua do Carmo in the centre of Porto is what appears to be a very large church with a split personality: the wild Rococco style on the right and the Classical style on the left. Look very carefully and you will see a section between the two halves, no more than a metre wide, with a doorway and windows, and it starts to become clear that these are in fact two separate churches separated by a third building. The Carmo Church (Igreja do Carmo) on the right was designed by the architect José Figueiredo Seixas in the 18th century (it was completed in 1768) for an order of Carmelite monks. On the side of the building is an impressive azulejo (tile panel), designed by Silvestro Silvestri, depicting the creation of the Carmelite community on Mount Carmel in Israel in the 13th century. The less-ostentatious Carmelite Church (Igreja das Carmelitas) on the left was completed in 1628 for an order of Carmelite nuns. The former convent is now used as the headquarters of the Porto branch of the National Republican Guard.
It is very unusual to see two churches next to each other in Portugal, despite the high number of churches in every town and city. The reason is that a law existed decreeing that churches could not share the same wall, so when these two churches were built a house, which must be one of the narrowest houses in the world, was built between them. What was the reason for this law? The most likely answer is that it was to prevent fraternization of the nuns and the monks. Despite the obvious limitations of the house, it remained lived in until the 1980s.
Igrega das Carmelitas and Igreja do Carmo, Rua do Carmo, Porto (nearest metro stations: São Bento and Aliados)