St. John's Church is located beneath Mainz Cathedral. This 1100-year-old church was the first episcopal see of the Bishop of Mainz. It is the oldest church in Mainz, the oldest cathedral in the Germany of today and the only preserved cathedral building from late Carolingian and early Ottonian time in Germany.
St. John's Church is predominantly Carolingian in style, but later exterior additions over many centuries have resulted in the appearance of various architectural influences seen today. It comprises three naves and stands under the patronage of John the Baptist today. It can be assumed that the church was dedicated initially to Martin of Tours since Martin is the Patron saint of the Roman Catholic Diocese of Mainz, then to St. Salvator, consecrated in 911 by Archbishop Hatto I and served as the cathedral for the Bishop of Mainz until the appointment of Willigis as Archbishop of Mainz in 975. It is documented that archbishop Erkanbald was buried in this first cathedral.
The church building dates from the early medieval era; construction of the main area of the church began in the 7th century. The successional building kept the guidelines of the floor plan of the original building. Knöchlein speculates that as the predecessors have also been made of stone, which is uncommon for the post-Roman period, leads to the conclusion that the early Christians in Mainz worshipped first in either a civilian building near the Forum or on the foundations of an ancient temple.
Archbishop Hatto's church is one of the few remaining late Carolingian churches. It was built as a basilica, its wide central nave is higher than the narrow aisles. The building was designed as a double chancel facility in east-west direction. Hatto has partially used wall remains of the first building. There are remains that have been integrated into the new church. In other places, this was completely demolished. The archeologists assume that it was a planned demolition and reconstruction - probably a smooth transition. The present floor level is probably the seventh level in total.
In the East the choir room is as wide as the nave. The side walls of the east choir are opened by two arched windows and overlying round windows (so-called Oeil-de-boeuf). West to the choir joins a rectangular nave flanked by narrow aisles. The walls of the nave are at the top of the clerestory broken by four arched windows, including four arcades open each (now closed) in the former aisles (now parish rooms).
References:The Clementinum is a historic complex of buildings in Prague. Until recently the complex hosted the National, University and Technical libraries, the City Library also being located nearby on Mariánské Náměstí. The Technical library and the Municipal library have moved to the Prague National Technical Library at Technická 6 since 2009. It is currently in use as the National Library of the Czech Republic.
Its history dates from the existence of a chapel dedicated to Saint Clement in the 11th century. A Dominican monastery was founded in the medieval period, which was transformed in 1556 to a Jesuit college. In 1622 the Jesuits transferred the library of Charles University to the Klementinum, and the college was merged with the University in 1654. The Jesuits remained until 1773, when the Klementinum was established as an observatory, library, and university by the Empress Maria Theresa of Austria.