Herbicides, also commonly known as weed killers, are substances used to control undesired plants, also known as weed. Selective herbicides control specific weed species while leaving the desired crop relatively unharmed, while non-selective herbicides (sometimes called total weed killers in commercial products) can be used to clear waste ground, industrial and construction sites, railways and railway embankments as they kill all plant material with which they come into contact. A great advantage of chemical herbicides over mechanical weed control is the ease of application, which often saves on the cost of labour.