The pons asinorum in Byrne's edition of the Elements showing part of Euclid's proof.

In geometry, the statement that the angles opposite the equal sides of an isosceles triangle are themselves equal is known as the pons asinorum (Latin: [ˈpõːs asɪˈnoːrũː], English: /ˈpɒnz ˌæsɪˈnɔːrəm/ PONZ ass-i-NOR-əm), typically translated as "bridge of asses". This statement is Proposition 5 of Book 1 in Euclid's Elements, and is also known as the isosceles triangle theorem. Its converse is also true: if two angles of a triangle are equal, then the sides opposite them are also equal. The term is also applied to the Pythagorean theorem.[1]

Pons asinorum is also used metaphorically for a problem or challenge which acts as a test of critical thinking, referring to the "ass' bridge's" ability to separate capable and incapable reasoners. Its first known usage in this context was in 1645.[2]

A persistent piece of mathematical folklore claims that an artificial intelligence program discovered an original and more elegant proof of this theorem.[3][4] In fact, Marvin Minsky recounts that he had rediscovered the Pappus proof (which he was not aware of) by simulating what a mechanical theorem prover might do.[5][6]


Proclus' proof
Euclid's Elements Book 1 proposition 5; the pons asinorum

Euclid and Proclus

Euclid's statement of the pons asinorum includes a second conclusion that if the equal sides of the triangle are extended below the base, then the angles between the extensions and the base are also equal. Euclid's proof involves drawing auxiliary lines to these extensions. But, as Euclid's commentator Proclus points out, Euclid never uses the second conclusion and his proof can be simplified somewhat by drawing the auxiliary lines to the sides of the triangle instead, the rest of the proof proceeding in more or less the same way.

There has been much speculation and debate as to why Euclid added the second conclusion to the theorem, given that it makes the proof more complicated. One plausible explanation, given by Proclus, is that the second conclusion can be used in possible objections to the proofs of later propositions where Euclid does not cover every case.[7] The proof relies heavily on what is today called side-angle-side, the previous proposition in the Elements.

Proclus' variation of Euclid's proof proceeds as follows:[8]

Let ABC be an isosceles triangle with AB and AC being the equal sides. Pick an arbitrary point D on side AB and construct E on AC so that AD = AE. Draw the lines BE, DC and DE.
Consider the triangles BAE and CAD; BA = CA, AE = AD, and is equal to itself, so by side-angle-side, the triangles are congruent and corresponding sides and angles are equal.
Therefore and , and BE = CD.
Since AB = AC and AD = AE, BD = CE by subtraction of equal parts.
Now consider the triangles DBE and ECD; BD = CE, BE = CD, and have just been shown, so applying side-angle-side again, the triangles are congruent.
Therefore and .
Since and , by subtraction of equal parts.
Consider a third pair of triangles, BDC and CEB; DB = EC, DC = EB, and , so applying side-angle-side a third time, the triangles are congruent.
In particular, angle CBD = BCE, which was to be proved.


Proclus gives a much shorter proof attributed to Pappus of Alexandria. This is not only simpler but it requires no additional construction at all. The method of proof is to apply side-angle-side to the triangle and its mirror image. More modern authors, in imitation of the method of proof given for the previous proposition have described this as picking up the triangle, turning it over and laying it down upon itself.[9][6] This method is lampooned by Charles Lutwidge Dodgson in Euclid and his Modern Rivals, calling it an "Irish bull" because it apparently requires the triangle to be in two places at once.[10]

The proof is as follows:[11]

Let ABC be an isosceles triangle with AB and AC being the equal sides.
Consider the triangles ABC and ACB, where ACB is considered a second triangle with vertices A, C and B corresponding respectively to A, B and C in the original triangle.
is equal to itself, AB = AC and AC = AB, so by side-angle-side, triangles ABC and ACB are congruent.
In particular, .[12]


A textbook proof

A standard textbook method is to construct the bisector of the angle at A.[13] This is simpler than Euclid's proof, but Euclid does not present the construction of an angle bisector until proposition 9. So the order of presentation of Euclid's propositions would have to be changed to avoid the possibility of circular reasoning.

The proof proceeds as follows:[14]

As before, let the triangle be ABC with AB = AC.
Construct the angle bisector of and extend it to meet BC at X.
AB = AC and AX is equal to itself.
Furthermore, , so, applying side-angle-side, triangle BAX and triangle CAX are congruent.
It follows that the angles at B and C are equal.

Legendre uses a similar construction in Éléments de géométrie, but taking X to be the midpoint of BC.[15] The proof is similar but side-side-side must be used instead of side-angle-side, and side-side-side is not given by Euclid until later in the Elements.

In 1876, while a member of the United States Congress, future President James A. Garfield developed a proof using the trapezoid, which was published in the New England Journal of Education.[16] Mathematics historian William Dunham wrote that Garfield's trapezoid work was "really a very clever proof."[17] According to the Journal, Garfield arrived at the proof "in mathematical amusements and discussions with other members of congress."[18]

In inner product spaces

The isosceles triangle theorem holds in inner product spaces over the real or complex numbers. In such spaces, it takes a form that says of vectors x, y, and z that if[19]




where θ is the angle between the two vectors, the conclusion of this inner product space form of the theorem is equivalent to the statement about equality of angles.

Etymology and related terms

Another medieval term for the pons asinorum was Elefuga which, according to Roger Bacon, comes from Greek elegia "misery", and Latin fuga "flight", that is "flight of the wretches". Though this etymology is dubious, it is echoed in Chaucer's use of the term "flemyng of wreches" for the theorem.[20]

There are two possible explanations for the name pons asinorum, the simplest being that the diagram used resembles an actual bridge. But the more popular explanation is that it is the first real test in the Elements of the intelligence of the reader and functions as a "bridge" to the harder propositions that follow.[21] Gauss supposedly once espoused a similar belief in the necessity of immediately understanding Euler's identity as a benchmark pursuant to becoming a first-class mathematician.[22]

Similarly, the name Dulcarnon was given to the 47th proposition of Book I of Euclid, better known as the Pythagorean theorem, after the Arabic Dhū 'l qarnain ذُو ٱلْقَرْنَيْن, meaning "the owner of the two horns", because diagrams of the theorem showed two smaller squares like horns at the top of the figure. The term is also used as a metaphor for a dilemma.[20] The theorem was also sometimes called "the Windmill" for similar reasons.[23]

Metaphorical usage

Uses of the pons asinorum as a metaphor for a test of critical thinking include:


  1. ^ Smith, David Eugene (1925). History Of Mathematics. Vol. II. Ginn And Company. pp. 284. It formed at bridge across which fools could not hope to pass, and was therefore known as the pons asinorum, or bridge of fools.¹
    1. The term is sometimes applied to the Pythagorean Theorem.
  2. ^ Pons asinorum — Definition and More from the Free Merriam
  3. ^ Jaakko Hintikka, "On Creativity in Reasoning", in Ake E. Andersson, N.E. Sahlin, eds., The Complexity of Creativity, 2013, ISBN 9401587884, p. 72
  4. ^ A. Battersby, Mathematics in Management, 1966, quoted in Deakin
  5. ^ Jeremy Bernstein, "Profiles: A.I." (interview with Marvin Minsky), The New Yorker December 14, 1981, p. 50-126
  6. ^ a b Michael A.B. Deakin, "From Pappus to Today: The History of a Proof", The Mathematical Gazette 74:467:6-11 (March 1990) JSTOR 3618841
  7. ^ Heath pp. 251–255
  8. ^ Following Proclus p. 53
  9. ^ For example F. Cuthbertson Primer of geometry (1876 Oxford) p. 7
  10. ^ Charles Lutwidge Dodgson, Euclid and his Modern Rivals Act I Scene II §6
  11. ^ Following Proclus p. 54
  12. ^ Heath p. 254 for section
  13. ^ For example J.M. Wilson Elementary geometry (1878 Oxford) p. 20
  14. ^ Following Wilson
  15. ^ A. M. Legendre Éléments de géométrie (1876 Libr. de Firmin-Didot et Cie) p. 14
  16. ^ G., J. A. (1876). "Pons Asinorum". New England Journal of Education. 3 (14): 161. ISSN 2578-4145. JSTOR 44764657.
  17. ^ Dunham, William (1994). The Mathematical Universe: An Alphabetical Journey Through the Great Proofs, Problems, and Personalities. Wiley & Sons. p. 99. ISBN 9780471536567.
  18. ^ Kolpas, Sid J. "Mathematical Treasure: Garfield's Proof of the Pythagorean Theorem". Mathematical Assoc. of America. Archived from the original on December 6, 2021. Retrieved December 22, 2021.
  19. ^ J. R. Retherford, Hilbert Space, Cambridge University Press, 1993, page 27.
  20. ^ a b c d A. F. West & H. D. Thompson "On Dulcarnon, Elefuga And Pons Asinorum as Fanciful Names For Geometrical Propositions" The Princeton University bulletin Vol. 3 No. 4 (1891) p. 84
  21. ^ D.E. Smith History of Mathematics (1958 Dover) p. 284
  22. ^ Derbyshire, John (2003). Prime Obsession: Bernhard Riemann and the Greatest Unsolved Problem in Mathematics. 500 Fifth Street, NW, Washington D.C. 20001: Joseph Henry Press. p. 202. ISBN 0-309-08549-7. first-class mathematician.((cite book)): CS1 maint: location (link)
  23. ^ Charles Lutwidge Dodgson, Euclid and his Modern Rivals Act I Scene II §1
  24. ^ W.E. Aytoun (Ed.) The poetical works of Thomas Campbell (1864, Little, Brown) p. 385 Google Books
  25. ^ John Stuart Mill Principles of Political Economy (1866: Longmans, Green, Reader, and Dyer) Book 2, Chapter 16, p. 261
  26. ^ Reid, Michael (28 October 2006). "Rubik's Cube patterns". Archived from the original on 12 December 2012. Retrieved 22 September 2019.
  27. ^ Eric S. Raymond, "Why Python?", Linux Journal, April 30, 2000
  28. ^ Aasinsilta on laiskurin apuneuvo | Yle Uutiset |