Black tea is a type of tea that is more oxidized than oolong, green and white teas. Black tea is generally stronger in flavor than the less oxidized teas. All four types are made from leaves of the shrub (or small tree) Camellia sinensis. Two principal varieties of the species are used – the small-leaved Chinese variety plant (C. sinensis subsp. sinensis), used for most other types of teas, and the large-leaved Assamese plant (C. sinensis subsp. assamica), which was traditionally mainly used for black tea, although in recent years some green and white have been produced. In Chinese and the languages of neighboring countries, black tea is known as "red tea" (Chinese 紅茶 hóngchá, pronounced [xʊ̌ŋʈʂʰǎ]; Japanese 紅茶 kōcha; Korean 홍차 hongcha, Bengali লাল চা Lal cha, Assamese ৰঙা চাহ Ronga chah), a description of the colour of the liquid; the Western term "black tea" refers to the colour of the oxidized leaves. In Chinese, "black tea" is a commonly used classification for post-fermented teas, such as Pu-erh tea; outside of China and its neighbouring countries, "red tea" more commonly refers to rooibos, a South African herbal tea. While green tea usually loses its flavor within a year, black tea retains its flavour for several years. For this reason, it has long been an article of trade, and compressed bricks of black tea even served as a form of de facto currency in Mongolia, Tibet and Siberia into the 19th century.[1] Although green tea has recently seen a revival due to its purported health benefits, black tea still accounts for over ninety percent of all tea sold in the West.