The practical operation of a channel is thought of as being continuous, with one word after another being sent. In fact, the reader has probably intuitively envisioned it as such, and nothing in the channels which we have so far explicitly studied is incompatible with this. Suppose, however, that a word is being sent after the channel has been in operation for some time. Then some channels may “remember” the past history of the channel, i. e., the words which were sent and received over it, and this memory of the past may affect the distribution of error for the word now being sent. In the next section of this chapter we will describe a reasonable model of a channel which has such a memory and is in continuous operation. Even without such a model the discussion of the present section will be completely intelligible ; its purpose is to explain the problems which arise for channels with memory for the past history and the restrictions which we shall impose. The reader impatient with such a discussion, or in no need of it, may prefer to proceed at once to the next section, and can do so without loss of logical continuity.