This paper concerns itself with binary coding and more specifically with an upper bound for the number of binary code works of lengthnsuch that every two differ in at leastdpositions from each other. It is shown that there is a relationship between this problem and a well-known extremum problem. The techniques developed for the latter problem are used to derive upper bounds for the former, one of which has been obtained in an entirely different way by Plotkin. IfB(n, d)denotes the maximum number of binary code words of lengthnand distanced, it is shown that \begin{equation} B(n,d) \leq \frac{2d}{2d-n} for d > n/2. \end{equation} The second bound due to Rankin is \begin{equation} B(n, d) \leq \frac{8d(n - d)}{n - (n- 2d)^2} for n - \sqrt{n}npositions. Although Rankin's bound contains the restriction that the binary code words consist of pairs, each pair differing innpositions, it is shown that the restriction may not be severe in the sense that the bound can often be attained.