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See also: isósceles


An isosceles triangle.
An isosceles trapezoid.


Borrowed from Latin īsoscelēs, from Ancient Greek ἰσοσκελής (isoskelḗs, equal-legged), from ἴσος (ísos, equal) +‎ σκέλος (skélos, leg) +‎ -ής (-ḗs, adjective suffix).


  • IPA(key): /aɪˈsɒsəliːz/
  • (file)


isosceles (not comparable)

  1. (geometry) Having (at least) two sides of equal length, used especially of a triangle or trapezoid.
    • 1693, Abel Swall, transl., The New Method of Fortification, as Practised by Monsieur de Vavban, Engineer General of France, 2nd edition, "A New Treatise of Fortification", page 96:
      Upon each exterior side draw an Isosceles Triangle of 480 Fathoms.
    • 1914, Henry Parker Manning, Geometry of Four Dimensions, page 204:
      A right double pyramid is isosceles when the extremities of the vertex-edge are at the same distance from the plane of the base.
    • 1945, Harold E. Wolfe, Introduction to Non-Euclidean Geometry, page 31:
      To prepare for the application of his method, Saccheri made use of a figure with which we are already acquainted. This is the isosceles quadrilateral with the two base angles right angles.
    • 1965 April 8, Newton B. Dismukes, “Multihull vessels (Patent US3316873A)”, in Google Patents[1]:
      The polygon advantageously is an isosceles trapezoid or rectangle with the pivotal connections between the deck or superstructure and four hulls respectively at its corners.
    • 2019 January 23, Altered Book Lover, “Funky Flowers”, in Blogspot[2]:
      The flowers were stuck inside a flower pot that was shaped like an isosceles polygon with a rectangular "edge."

Usage notes[edit]

  • A trapezoid with two equal sides and two unequal sides is normally described as unequilateral, even though it can be regarded as a special case of isosceles trapezoid.
  • A triangle with three equal sides is normally described as equilateral, even though it can be regarded as a special case of isosceles triangle.

Derived terms[edit]


Further reading[edit]




Borrowed from Ancient Greek ἰσοσκελής (isoskelḗs), from ἴσος (ísos, equal) +‎ σκέλος (skélos, leg) +‎ -ής (-ḗs, adjective suffix).



īsoscelēs (genitive īsoscelis); third-declension one-termination adjective

  1. (geometry) isosceles (having equal legs)


Third-declension one-termination adjective (Greek-type).

Number Singular Plural
Case / Gender Masc./Fem. Neuter Masc./Fem. Neuter
Nominative īsoscelēs īsosceles1
īsoscelēs īsoscelia
Genitive īsoscelis īsoscelium
Dative īsoscelī īsoscelibus
Accusative īsoscelem īsosceles1
īsoscelēs īsoscelia
Ablative īsoscelī īsoscelibus
Vocative īsosceles1
īsoscelēs īsoscelia

1It is unknown if Classical Latin preserved (or would have preserved) the shortness of the original Greek short ending. Notes:

  • The Greek masculine and feminine nominative singular is ἰσοσκελής (isoskelḗs), while the masculine and feminine vocative singular and the neuter nominative, accusative and vocative singular are ἰσοσκελές (isoskelés). Maybe Latin preserved the short length of the epsilon (ε), or maybe it did not so that the declension became similar to Latin third declension adjectives of one ending (like felix).
  • This word is often used together with triangulum n and rarer with triangulus m.


  • īsoscĕles”, in Charlton T. Lewis and Charles Short (1879) A Latin Dictionary, Oxford: Clarendon Press
  • īsoscĕlēs in Gaffiot, Félix (1934) Dictionnaire illustré latin-français, Hachette, page 860/3