The most significant digit of a number written in a given positional base is the digit in the most significant place value, and has to be in the range . In the case of an integer, the most significant digit is the 's place value, where is the total number of digits, or .

In an array of digits long meant for mathematical manipulation, it might be convenient to index the least significant digit with index 1 or 0, and the more significant digits with larger integers. (This enables the calculation of the value of a given digit as rather than .) For an array of digits meant for text string manipulation, however, the most significant digit might be placed at position 0 or 1 (for example, by Mathematica's IntegerDigits function).

In binary, the most significant digit is often called the most significant bit.