ETAOIN SHRDLU is the approximate order of frequency of the twelve most common letters in the English language. The ordering originated in the era of "hot type" publishing as a nonsensical phrase used by Linotype operators to indicate that an error had occurred.
The Linotype keyboard was vertically aligned by frequency. As a result, its two left-most columns formed the phrase ETAOIN SHRDLU. In addition, the Linotype machine could not "backspace." Consequently, operators who made a mistake needed to finish the now erroneous line of text before they could eject the slug and start anew. Since the slug already contained errors, its remaining content did not matter. The quickest way to finish the line—that is, to complete the mistake—was to run one's fingers down the left-most columns of the keyboard, an act that repeatedly printed the phrase ETAOIN SHRDLU. If the erroneous slug was not ejected, its distinctive set of letters helped compositors quickly identify it for removal. Occasionally, however, compositors overlooked the phrase and so unknowingly printed it.