Plant hybrids, especially, are often stronger than either parent variety, a phenomenon which when present is known as hybrid vigor (heterosis) or heterozygote advantage. Plant breeders make use of a number of techniques to produce hybrids, including line breeding and the formation of complex hybrids. An economically important example is hybrid maize (corn), which provides a considerable seed yield advantage over open pollinated varieties. Hybrid seed dominates the commercial maize seed market in the United States, Canada and many other major maize producing countries.