Middle Ages:

The importance of infantry and light cavalry began to decline during the early Carolingian period, with a growing dominance of elite heavy cavalry. The use of militia-type levies of the free population declined over the Carolingian period. Although much of the Carolingian armies were mounted, a large proportion during the early period appear to have been mounted infantry, rather than true cavalry. One exception was Anglo-Saxon England, where the armies were still composed of regional levies, known as the fyrd, which were led by the local elites. In military technology, one of the main changes was the return of the crossbow, which had been known in Roman times and reappeared as a military weapon during the last part of the Early Middle Ages. Another change was the introduction of the stirrup, which increased the effectiveness of cavalry as shock troops. A technological advance that had implications beyond the military was the horseshoe, which allowed horses to be used in rocky terrain.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". Along with the horseshoe, what invention important for cavalry emerged in this period?
the stirrup