The Case of the "Flitterbat Lancers" by Arthur Morrison

"The meaning of the thing was so very plain," Hewitt said to me afterwards, "that the duffers who had the ‘Flitterbat Lancers' in hand for so long never saw it at all. If Shiels had made an ordinary clumsy cryptogram, all letters and figures, they would have seen what it was at once, and at least would have tried to read it; but because it was put in the form of music, they tried everything else but the right way. It was a clever dodge of Shiels's, without a doubt. Very few people, police officers or not, turning over a heap of old music, would notice or feel suspicious of that little slip among the rest. But once one sees it is a cryptogram (and the absence of bar-lines and of notes beyond the stave would suggest that) the reading is as easy as possible. For my part I tried it as a cryptogram at once. You know the plan -- it has been described a hundred times.

See here -- look at this copy of the ‘Flitterbat Lancers.' Its only difficulty -- and that is a small one -- is that the words are not divided. Since there are on the stave positions for less than a dozen notes, and there are twenty-six letters to be indicated, it follows that crotchets, quavers, and semiquavers on the same line or space must mean different letters. The first step is obvious. We count the notes to ascertain which sign occurs most frequently, and we find that the crotchet in the top space is the sign required -- it occurs no less than eleven times. Now the letter most frequently occurring in an ordinary sentence of English is _e_. Let us then suppose that this represents _e_. At once a coincidence strikes us. In ordinary musical notation in the treble clef the note occupying the top space would be E. Let us remember that presently.