diagonal

Etymology

It is borrowed from Latin diagōnālis,^[2][3][4][5] a word used by Vitruvius;^[2] in Barnhart, Etymonline and Skeat it is borrowed from Middle French diagonal, from the Latin.^[6][7][8] The French word is recorded from the 13th century.^[2] The Latin adjective is from diagōnus (“slanting line”),^[6][7] from Ancient Greek διαγώνιος (diagṓnios, “from angle to angle”),^[2][3][9] formed from διά (diá, “across, through”) and γωνία (gōnía, “angle, corner”),^[2][7][9] as in polygon.^[9] The Greek word for angle is related to γόνυ (gónu, “knee”) and so to English knee and genuine;^[6][10][9] Etymonline traces it to Proto-Indo-European *genu- (“knee; angle”).^[7]

Earliest recorded meaning

Of a line, running from one angle of a figure to an opposite, not adjacent angle: the OED's first record is from about 1541, in Copland's translation of Guy de Chauliac's surgery;^[2] Barnhart dates the adjective to 1563, implied earlier in the adverb diagonally, in 1541 and in a Middle English translation of Chauliac's Grande Chirurgie probably before 1425.^[6]

Semantic evolution

Ayto notes that the strict geometric meaning, a line joining two angles of a figure that are not next to each other, shows the origin better than the common loose use for "oblique".^[9] The noun, a diagonal line, is recorded from 1571, in a geometry by Digges.^[2][7][10] The looser sense of slanting or oblique followed in 1665,^[2] and the sense "marked with oblique lines" before 1679.^[2] Johnson (1755) defined the adjective as reaching from one angle to another so as to divide a parallelogram into equal parts.^[11] The noun was later applied to a row of squares in chess,^[7][12] to a beam or plank set slantwise in a structure (1837),^[2] and to a cloth woven with diagonal ribs (1861).^[2][12]

Timeline

• before 1425 – implied in the adverb diagonally.^[6]
• c. 1541 – of a line, joining non-adjacent angles of a figure.^[2]
• 1563 – the adjective, in Barnhart's dating.^[6][10]
• 1571 – noun, a diagonal line.^[2][10]
• 1665 – slanting, oblique.^[2]
• before 1679 – marked with oblique lines.^[2]
• 1837 – a beam or plank set diagonally in a structure.^[2]
• 1861 – a cloth woven with diagonal ribs.^[2]

Meanings

Adjective

1. (mathematics) Joining two vertices of a polygon that are not next to each other, or two vertices of a polyhedron not on the same face.^[12][10][2]
2. Running at a slant; oblique: The path cut a diagonal line across the lawn.^[12][10][2]
3. Marked with slanting lines or patterns: a diagonal weave.^[12][2]

Noun

1. (mathematics) A diagonal line or plane.^[12][10][2]
2. (chess) A slanting row of squares of the same colour.^[12][7]
3. A slanting direction, row or pattern.^[10]
4. A cloth woven with slanting ribs, such as twill.^[12][2]
5. A part of a structure, such as a beam, set slantwise.^[2]

Word family

• diagonally
• diagonalize
• nondiagonal
• diagonal matrix
• main diagonal

Related words

• polygon
• knee
• genuine
• trigonometry

References

1. ^ a b Wiktionary, the free dictionary, https://en.wiktionary.org
2. ^ a b c d e f g h i j k l m n o p q r s t u v w Oxford English Dictionary Online (accessed March 2026), Oxford: Oxford University Press
3. ^ a b The American Heritage Dictionary of the English Language (online 2024), 5th ed., Boston: Houghton Mifflin Harcourt
4. ^ Hoad, T. F., ed. (1986), The Concise Oxford Dictionary of English Etymology, Oxford: Clarendon Press
5. ^ Stevenson, Angus, ed. (2010), Oxford Dictionary of English, 3rd ed., Oxford: Oxford University Press
6. ^ a b c d e f Barnhart, Robert K. (1988), The Barnhart Dictionary of Etymology
7. ^ a b c d e f g Harper, Douglas. Online Etymology Dictionary, https://www.etymonline.com
8. ^ Skeat, Walter W. (1911), A Concise Etymological Dictionary of the English Language, new ed., Oxford: Clarendon Press
9. ^ a b c d e Ayto, John (2005), Word Origins: The Secret Histories of English Words from A to Z, 2nd ed., London: A & C Black
10. ^ a b c d e f g h Merriam-Webster Unabridged Dictionary (2024), online ed., Springfield, Mass.: Merriam-Webster
11. ^ Johnson, Samuel (1755), A Dictionary of the English Language, London; 4th ed. 1773; Johnson's Dictionary Online, ed. Beth Rapp Young et al.
12. ^ a b c d e f g h Collins English Dictionary (2023), Glasgow: HarperCollins