The dental operatory, or treatment room, is the work area of the dentist and clinical assistant. It is designed for speed, efficiency, and comfort. Common operatory equipment includes the dental chair, dental unit, dental operating light, x-ray machine, operating stools, mobile carts, and cabinets. High-tech tools include air abrasion cavity preparation systems, lasers, CAD/CAM systems, computers and associated equipment, curing lights, digital radiographic equipment, and intraoral cameras. In four-handed dentistry, dentists work with one or more clinical assistants positioned around the patient using instruments arranged on a specially designed pre-set tray in their sequence of use. Four-handed dentistry takes into consideration ergonomics, or the physical relationship between people and their environment. It is very important in the dental operatory to adapt equipment and instruments to the worker. Four-handed dentistry is based on the principles of motion economy, which eliminates or minimizes the number and length of motions used at chairside. There are four zones of activity around which the dental team works and three basic systems for instrument delivery.