Short dynamic poses are used for gesture drawing exercises or warm-ups, with the model taking strenuous or precarious positions that could not be sustained for a longer pose—just long enough for the artist to quickly capture the essence of it. Drawing sessions proceed through groups of 5, 10, 15, or 20 minute poses generally for a total of three hours. Active, gestural, or challenging standing poses are often scheduled at the beginning of a session when the models' energy level is highest. Specific exercises or lesson plans may require a particular type of pose, but more often the model is expected to do a series of poses with little direction. The more a model knows about the types of exercises used to teach art, the better they become at posing. Poses fall into three basic categories: standing, seated and reclining. Within each of these, there are varying levels of difficulty, so one kind is not always easier than another. Artists and life drawing instructors will often prefer poses in which the body is being exerted, for a more dynamic and aesthetically interesting subject. Common poses such as standing twists, slouched seated poses and especially the classical contrapposto are difficult to sustain accurately for any amount of time, although it is often surprising what a skilled model can do. The model's level of experience and skill may be taken into account in determining the length of the posing session and the difficulty of the poses. While posing, a model is expected to remain essentially motionless, and usually remains silent. Exact requirements may vary depending upon the artist or instructor, but an experienced model will not speak, wriggle, scratch, or readjust during the pose. To accommodate the physical limitations, a schedule such as 20 to 25 minutes of posing, with five- or ten-minute breaks is observed. These breaks—during which the model usually wears a robe or puts on clothing—allow the model to stretch, relax and attend to other needs. In art schools classrooms or studios, the model usually poses on a raised platform called the model stand or dais. In sculpture studios this platform may be built to rotate to allow for a 360° view for every artist. Long poses are generally required for painting (hours or days) and sculpture (perhaps days). To aid in resuming a long pose after a break, chalk marks and/or masking tape are often placed on the model stand. Models for life drawing classes usually pose nude, though visually non-obstructive personal items such as small jewelry and eyeglasses may be worn.