Scenery changes are sometimes made mid-scene, while the actors remain on stage and the curtain stays open. This is sometimes accomplished by using a , or "small wagon stage". This technique originated at the beginning of the 18th century, where scenery or actors move on or off stage on a wheeled platform. Also common are stagehands rushing onto the stage adding and removing props, backdrops and other scenery; these are always dressed entirely in black and are traditionally considered invisible. Stagehands also assist in a variety of quick costume changes known as ("quick change technique").