"One billion coins is referred to as 1bil/1bill or 1B. No stack can hold more than 2,147,483,647 (2^31-1) coins in one stack. This is because RuneScape is a java based game, and the number of coins in a stack is an "int" variable, whose 32-bit values can range only from -2,147,483,647 to 2,147,483,647, and can only be integers." - Off the RSWiki.