The most significant digit of a number $n$ written in a given positional base $b$ is the digit in the most significant place value, and has to be in the range $-1 < d_k < b$ . In the case of an integer, the most significant digit is the $b^k$ 's place value, where $k$ is the total number of digits, or $k = \lfloor log_{b} n \rfloor$ .

In an array of digits $k$ 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 $d_ib^i$ rather than $d_ib^{k - i}$ .) 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.