Rare earth elements (REEs) consist of 15 lanthanides, yttrium and scandium, which can be categorized into light rare earth elements (LREEs) and heavy rare earth elements (HREEs). Historically, prices of individual elements have not always moved concurrently, with prices of HREEs higher than those of LREEs. Prices for many of these elements increased in the 1960s and the 1970s but remained stable in the 1980s.