There were multiple systems One (mentioned already in another answer) was to put a bar (called vinculum or titulum) above the digit to show that this digit was multiplied with 1. The vinculum counts only for one digit alone and not for a whole number. As sometimes bars were also used to distinguisch numbers from charactersequences that could be ambigous sone times. Another was putting a frame (may be closed open at the bottom or just the left and right pilar) to tell the reader that this digit is to be multiplied with 1. This could be done for numbers and single digits so |MM| would have been 2 Millions. That frame was considered more easier to apply than the apostrophus. Another style us the previously mentioned apostrophus. The original sign for 1 was PHI math a character that can also be seen as the sequence (|). That left some possibilities for playing using only the right half of it you got D (=5) adding one opening and closing sign was multiplying by 1 so ((|)) would be 1 add 2 more open and close you get a million ((((|)))). Framing is the easier applieable method.