Conca di Viarenna, Navigation lock in Milan, Italy

Conca di Viarenna is a dry navigation lock on Via Conca del Naviglio, built by the Veneranda Fabbrica del Duomo between 1551 and 1558 to replace a 1438 lock demolished during construction of Milan's Spanish walls. It overcame a roughly two-metre drop between the Cerchia dei Navigli and the Darsena di Porta Ticinese along the Naviglio Vallone. At one end stands a marble aedicule with the 1497 ducal decree exempting Cathedral-bound barges from tolls.

Built between 1551 and 1558 by the workshops overseeing Milan Cathedral, this lock replaced an earlier structure from 1439. Construction occurred during Spanish rule when the city was strengthening its fortifications.

At one end of the dry lock stands a shrine moved there from its original side, bearing a 1497 ducal decree carved on a Candoglia marble slab. It exempted boats carrying Cathedral materials from tolls with the formula AUF, Ad Usum Fabricae, from which the Italian phrase 'a uffa' or 'a ufo' meaning 'free of charge' derives. Above it is the split coat of arms of Ludovico il Moro and Beatrice d'Este, the same as above Leonardo's Last Supper.

The lock stands isolated from its original canal system since the canals were covered with asphalt in the 1930s. You can easily reach it on foot and visit it as part of a walk through the city.

The 1497 ducal decree exempting Cathedral boats from tolls is carved on a Candoglia marble plaque set in the shrine at one end of the lock. Its formula AUF, for Ad usum fabricae, gave Milanese the phrase 'a uffa' or 'a ufo', meaning free of charge.