Simplify the following text for a Grade 6 native English-speaking person.
Original: The original French metric system used the litre as a base unit , and it has been used in several subsequent versions of the metric system and is accepted for use with the SI , although not an official SI unit -- the SI unit of volume is the cubic metre ( m3 ) . The spelling of the word used by the International Bureau of Weights and Measures is `` litre '' and this is also the usual one in most English-speaking countries , but in American English the spelling is `` liter '' , being endorsed by the United States .
Simplified: A litre ( liter in the United States ) is the metric unit of volume .