The book ratio could be the small small small fraction of total build up that a bank keeps on hand as reserves (in other words. Money in the vault). Theoretically, the book ratio may also just take the type of a needed book ratio, or even the small fraction of deposits that the bank is needed to carry on hand as reserves, or a extra reserve ratio, the small small fraction of total deposits that the bank chooses to help keep as reserves far above exactly exactly what it really is necessary to hold.