Between the 9th and 14th centuries, the Muslim world developed many advanced economic concepts, techniques and usages. These ranged from areas of production, investment, finance, economic development, taxation, property use such as Hawala: an early informal value transfer system, Islamic trusts, known as waqf, systems of contract relied upon by merchants, a widely circulated common currency, cheques, promissory notes, early contracts, bills of exchange, and forms of commercial partnership such as mufawada.