Unlike the more familiar vertical waterwheel, the horizontal-wheeled mill, sometimes called a Norse or tub mill, worked on a simpler principle: water was directed through a chute or millrace onto flat or angled paddles arranged horizontally beneath the millstone, causing the entire shaft and stone to rotate together without the need for gearing. This made such mills cheap and practical to build, well suited to the fast-running streams of upland and boggy terrain, and they were widespread across early medieval Ireland before more complex vertical-mill technology became dominant.