JACA, a frontier city of Spain, in the province of Huesca, formerly capital of a partido in the kingdom of Aragon, is situated on the left bank of the Aragon, at an elevation of 2333 feet above the sea-level. It is the seat of a bishop, and the most important of the public buildings is the massive cathedral, the construction of which was begun under King Ramiro in 1040. The industries of the city are unimportant. Its population in 1877 was 4155.

The diligence road from Zaragoza to Pau by the Puerto de Canfranc passes through Jaca.

The origin of the city is unknown. The Jaccctani clairtcnravo() are mentioned as one of the most celebrated of the numerous small tribes inhabiting the basin of the Ebro by Strabo (p. 161), who adds that their territory was the theatre of the wars between Sertorius and Pompey, and afterwards between Pompey's son Sextus and the generals of Cxsar. They are probably identical with the Lacetani of Livy (xxi. 60, 61) and Cnsar (B.C., i. 60). Jaca at an early period of the invasion fell into the possession of the Moors, by whose writers it is referred to under the name of Dyaka as one of the chief places in the province of Sarkosta (Zaragoza). The date of its reconquest is uncertain, but it must have been before the time of Ramiro, who gave it the title of " city," and in 1063 held within its 'walls a council, which, inasmuch as the people were called in to sanction its decrees, is regarded as having been of great importance in the history of the parliamentary institutions of the peninsula. The original " fuero " of Jaca is one of the oldest extant. In 1705 Jaca was the only city which stood out for King Philip, from whom, in consequence, it received the title of "muy noble, muy leal y vencedora." In the war of independence in 1809 it surrendered to the French ; it also yielded to General Mina in 1814.