The tilt-table is a bed-like platform that tilts from the horizontal position. Head-down tilt (HDT) is the position whereby the bed is moved from the horizontal position so that the head is at a level lower than the feet. This can be at varying angles but the most commonly used angle is an HDT of 6 degrees. The HDT position can be used as a form of microgravity simulation for differing periods of time, varying from a few minutes to a few months, depending on the study protocol. It is used to simulate some of the effects that microgravity can have on human physiology. Some tilt-tables also combine the possibility of having a head-up tilt position in which the volunteer is tilted from the horizontal position so that the head is at a level higher than the feet. This can be used to test the physiological reactions to orthostasis after periods of bed rest (horizontal position) or HDT.