These notes explain, for specific variants and classes of variants, how the documentary evidence has been analyzed to identify active authorial intention or revision, or (conversely) to detect errors, inadvertent changes, and changes by typists, compositors, and proofreaders who intended to “improve” the text. The notes also report or describe authorial revision not fully accounted for in Alterations in the Manuscript. When the first American edition (hereafter “first edition”) of Adventures of Huckleberry Finn is cited as the source of an emendation, all extant forms of that typesetting agree (that is, the proofs, the salesmen’s prospectus, books issued under the Webster company imprint, and books issued in Canada from duplicate plates under the Dawson company imprint). Similarly, when the first American edition (hereafter “first edition”) of Life on the Mississippi is cited, all extant forms of that typesetting agree. For the convenience of the reader who wishes to consult the manuscript, where the manuscript is extant it is cited parenthetically following the page and line cue to the text of this edition, thus: “13.34 (MS1a, 43.9–11).” All of the manuscript facsimiles reproduced in the notes are by courtesy of the Buffalo and Erie County Public Library (NBuBE), which houses the original manuscript in the Mark Twain Room. A photofacsimile of MS2 is available in two-volume book form (SLC 1983), and a photofacsimile of the complete manuscript (MS1 and MS2) is available in “Adventures of Huckleberry Finn”: The Buffalo & Erie County Public Library CD-ROM Edition (Doyno 2003).