More expensive "waveless" modern waterbeds have a mixture of air and water chambers, usually interconnected. Waterbeds primarily consist of two types, hard-sided beds and soft-sided beds. Waterbeds are usually constructed from soft polyvinyl chloride (PVC) or similar material. Some can be inflated with liquids, such as waterbeds and water balloons. Waterbeds intended for medical therapies appear in various reports through the 19th century. The resulting slack allowed for easy adaptation for narrow and windy waterbeds.