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Borrowed from Latin separatrix, the feminine form of separator (originally owing to an implied līnea (line)), from sēparāre (to divide; to separate) + -trix (forming female agents). First developed as a decimal mark among the medieval Arab mathematicians, whence a shorter variant gave rise to the decimal comma employed by many European countries and their former colonies.


separatrix (plural separatrixes or separatrices)

  1. (typography, historical) The ⟨L⟩ or pipe|mark formerly used to divide integers from decimals.
    • 1660, Jonas Moore, Moor's Arithmetick, page 13:
      Therefore in writing of decimall parts let the separatrix be always used.
  2. (typography, obsolete) Synonym of decimal point, which replaced such marks.
  3. (typography) The proofreader's mark resembling a slash ⟨ / ⟩ or vertical bar ⟨ | ⟩ placed after a note in the margin to indicate that it should replace the item(s) struckthrough in the running text or to separate it from other margin notes.
  4. A terminator: a line on a partially-illuminated surface separating the lit and shaded regions.
  5. (physics) The line between regions having different magnetic fields.
  6. (mathematics) The boundary separating two modes of behavior in a differential equation.



  • Merriam-Webster Online. "separatrix". Accessed 11 February 2016.
  • Oxford English Dictionary, 1st ed. "separatrix, n." Oxford University Press (Oxford), 1912.