Wikipedia
The curtal sonnet is a form invented by Gerard Manley Hopkins, and used in three of his poems.
It is an eleven-line (or, more accurately, ten-and-a-half-line) sonnet, but rather than the first eleven lines of a standard sonnet it consists of precisely ¾ of the structure of a Petrarchan sonnet shrunk proportionally. The octave of a sonnet becomes a sestet and the sestet a quatrain plus an additional "tail piece." That is, the first eight lines of a sonnet are translated into the first six lines of a curtal sonnet and the last six lines of a sonnet are translated into the last four and a half lines of a curtal sonnet. Hopkins describes the last line as half a line, though in fact it can be shorter than half of one of Hopkins's standard sprung rhythm lines. In the preface to his Poems (1876-89), Hopkins describes the relationship between the Petrarchan and curtal sonnets mathematically; if the Petrarchan sonnet can be described by the equation 8+6=14 then, he says, the curtal sonnet would be:
$${12\over2}+{9\over2}={21\over2}=10{1\over2}$$
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Hopkins's only examples of the form are " Pied Beauty," "Peace," and "Ash Boughs." "Pied Beauty" is as follows, showing the proportional relation to the Petrarchan sonnet (not included in the original: the only indication of the form is in the preface). Accents indicate stressed syllables:
Glory be to God for dappled things— For skies of couple-colour as a brinded cow; For rose-moles all in stipple upon trout that swim; Fresh-firecoal chestnut-falls; finches' wings; Landscape plotted and pieced—fold, fallow, and plough; And áll trádes, their gear and tackle and trim.${12\over2}=6$
All things counter, original, spare, strange; Whatever is fickle, freckled (who knows how?) With swift, slow; sweet, sour; adazzle, dim; He fathers-forth whose beauty is past change: Praise him.${9\over2}=4{1\over2}$
Hopkins's account of the form comes from the preface to his Poems (1876-89). Critics are generally in agreement that the curtal sonnet does not so much constitute a new form as an interpretation of sonnet form as Hopkins believed it to be; as Elisabeth Schneider argues, the curtal sonnet reveals Hopkins's intense interest in the mathematical proportions of all sonnets. For an in-depth treatment of all three poems, see Lois Pitchford. The form has been used occasionally since, but primarily as a novelty, in contrast to Hopkins's quite serious use.
Usage examples of "curtal sonnet".
The words CURTAL SONNET had flashed before his, Hogg's, Enderby's, eyes, and the rhymes had lined up for inspection.