in blank

from The Collaborative International Dictionary of English v.0.48
Blank \Blank\, n.
   1. Any void space; a void space on paper, or in any written
      instrument; an interval void of consciousness, action,
      result, etc; a void.
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            I can not write a paper full, I used to do; and yet
            I will not forgive a blank of half an inch from you.
                                                  --Swift.
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            From this time there ensues a long blank in the
            history of French legislation.        --Hallam.
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            I was ill. I can't tell how long -- it was a blank.
                                                  --G. Eliot.
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   2. A lot by which nothing is gained; a ticket in a lottery on
      which no prize is indicated.
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            In Fortune's lottery lies
            A heap of blanks, like this, for one small prize.
                                                  --Dryden.
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   3. A paper unwritten; a paper without marks or characters a
      blank ballot; -- especially, a paper on which are to be
      inserted designated items of information, for which spaces
      are left vacant; a bland form.
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            The freemen signified their approbation by an
            inscribed vote, and their dissent by a blank.
                                                  --Palfrey.
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   4. A paper containing the substance of a legal instrument, as
      a deed, release, writ, or execution, with spaces left to
      be filled with names, date, descriptions, etc.
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   5. The point aimed at in a target, marked with a white spot;
      hence, the object to which anything is directed.
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            Let me still remain
            The true blank of thine eye.          --Shak.
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   6. Aim; shot; range. [Obs.]
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            I have stood . . . within the blank of his
            displeasure
            For my free speech.                   --Shak.
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   7. A kind of base silver money, first coined in England by
      Henry V., and worth about 8 pence; also, a French coin of
      the seventeenth century, worth about 4 pence. --Nares.
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   8. (Mech.) A piece of metal prepared to be made into
      something by a further operation, as a coin, screw, nuts.
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   9. (Dominoes) A piece or division of a piece, without spots;
      as, the "double blank"; the "six blank."
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   {In blank}, with an essential portion to be supplied by
      another; as, to make out a check in blank.
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from Bouvier's Law Dictionary, Revised 6th Ed (1856)
IN BLANK. This is generally applied to indorsements, as, indorsements in 
blank, which is one not restricted, made by the indorser simply writing his 
name. See Indorsement. 
    

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