Series or flight of steps that provides a means of moving from one level to another. The earliest stairways seem to have been built with walls on both sides, as in Egyptian pylons dating from the 2nd millennium BC. The Romans were noted for their monumental stairs. The modern use of steel and reinforced concrete has made possible the curves and sweeps of contemporary design. Staircases have traditionally been built of wood, marble or stone, and iron or steel. A step's horizontal surface is called its tread, the vertical front its riser. Traditional wooden staircases are constructed with stringers, beams inclined to the angle of the staircase. Stringers are supported by newel posts, which also support the handrail, forming a balustrade.