The hartley, also called a ban, or a dit, is a logarithmic unit that measures information or entropy, based on base 10 logarithms and powers of 10. One hartley is the information content of an event if the probability of that event occurring is ⁠1/10⁠. It is therefore equal to the information contained in one decimal digit, assuming a priori equiprobability of each possible value. It is named after Ralph Hartley.