1/2 Real of 10 Soldos - João I Porto mint

João I's monetary reforms of the early 1390s were driven largely by the financial strain of maintaining the Anglo-Portuguese alliance cemented at Windsor in 1386 — troops, garrisons, and diplomacy all demanded coin. The Porto mint, historically subordinate to Lisbon, was pressed into expanded production during this window precisely because the north required its own supply of petty coinage for day-to-day commerce independent of the capital's output.

The billon content here — barely eight percent silver — reflects chronic bullion shortages that plagued the Portuguese crown throughout João's reign and forced repeated debasements of the lower denominations.