A candle is an ignitable wick embedded in wax or another flammable solid substance such as tallow that provides light, and in some cases, a fragrance. It can also be used to provide heat, or used as a method of keeping time. A candle manufacturer is traditionally known as a chandler. Various devices have been invented to hold candles, from simple tabletop candle holders to elaborate chandeliers. For a candle to burn, a heat source (commonly a naked flame) is used to light the candle's wick, which melts and vaporizes a small amount of fuel (the wax). Once vaporized, the fuel combines with oxygen in the atmosphere to ignite and form a constant flame. This flame provides sufficient heat to keep the candle burning via a self-sustaining chain of events: the heat of the flame melts the top of the mass of solid fuel; the liquefied fuel then moves upward through the wick via capillary action; the liquefied fuel finally vaporizes to burn within the candle's flame. As the mass of solid fuel is melted and consumed, the candle becomes shorter. Portions of the wick that are not emitting vaporized fuel are consumed in the flame. The incineration of the wick limits the exposed length of the wick, thus maintaining a constant burning temperature and rate of fuel consumption. Some wicks require regular trimming with scissors (or a specialized wick trimmer), usually to about one-quarter inch (~0.7 cm), to promote slower, steady burning, and also to prevent smoking. In early times, the wick needed to be trimmed quite frequently. Special candle-scissors, referred to as "snuffers" were produced for this purpose in the 20th century and were often combined with an extinguisher. In modern candles, the wick is constructed so that it curves over as it burns. This ensures that the end of the wick gets oxygen and is then consumed by fire—a self-trimming wick.