Sign-value Notation
In sign-value notation systems like Egyptian numerals, where order is irrelevant, one might say that all characters are in the units place. In such a case, the quantity and range of characters permitted in the units place is potentially infinite and the face value is simply the sum of the values of all characters inserted. In sign-value notation systems where order is important like Roman numerals, only the characters that directly precede smaller value character, including the last one that precedes no character, may be said to be in the units place. Characters that directly precede a larger value character may be said to be in the reverse units place as they represent multiples of −1 (negative one).
Read more about this topic: Units Place