1 Real of 10 Reais Brancos- João I Porto mint

João I's reign saw Portugal pivot decisively toward Atlantic expansion — Ceuta fell in 1415, the same year this type begins — and the Crown's need to finance naval infrastructure and garrison costs drove sustained pressure on the silver coinage. The Porto mint operated as a secondary striking authority throughout this period, its output distinguishable from Lisbon pieces primarily through mint marks rather than any die quality differential.

The denomination itself, reckoned at 10 reais brancos, reflects a monetary accounting system anchored to the real branco rather than to the coin in hand — a abstraction that repeatedly confused foreign merchants trading into Lisbon's waterfront markets.