The need for unified notation in atmospheric and oceanic data assimilation arises from the field's rapid theoretical expansion and the desire to translate it into practical applications. Self-consistent notation is proposed that bridges sequential and variational methods, on the one hand, and operational usage, on the other. Over various other mottoes for this risky endeavor, the authors selected: "When I use a word, " Humpty Dumpty said, in rather a scornful voice tone, "it means just what I choose it to mean - neither more more less. "Lewis Carroll, 1871.