Complex sentence: Historically, banks sought to ensure that they could always had pay customers in coins when they presented banknotes for payment and this practice of "backing" notes with something of substance is the basis for the history of central banks backing their currencies in gold or silver.
Split into simpler sentences: Historically, banks sought to ensure that they could always pay customers in coins when they presented banknotes for payment. This practice of "backing" notes with something of substance is the basis for the history of central banks backing their currencies in gold or silver.