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Diagonal in Irish (Fiarthrasnán): Meaning, Pronunciation, Usage

The Irish for diagonal is fiarthrasnán, a masculine noun (plural fiarthrasnáin).
Updated October 2026Report a mistake

How to say “diagonal” in Irish

Irish:
fiarthrasnán
English:
diagonal
Word type:
noun
Gender:
masculine, first declension
Pronunciation:
“FEER-hras-nawn”  /ˈfʲiəɾˌhɾasˠn̪ˠaːnˠ/
Various forms
Plural:
fiarthrasnáin (diagonals)
Genitive singular:
fiarthrasnáin, an fhiarthrasnáin (e.g. fad an fhiarthrasnáin, length of the diagonal)
Genitive plural:
fiarthrasnán, na bhfiarthrasnán (e.g. líon na bhfiarthrasnán, number of diagonals)

What does “fiarthrasnán” mean?

diagonal
A straight line joining two opposite corners of a shape, running slantwise across it.Also: diagonal line, slanting line.

“Fiarthrasnán” with the article

The plural of diagonal in Irish is fiarthrasnáin. Here is fiarthrasnán after the article (an t-alt), in the singular and the plural.

an fiarthrasnán
the diagonal
No change after an
na fiarthrasnáin
the diagonals
Plural: no change after na

“Fiarthrasnán” in a sentence: Irish examples

Common Irish sentences using fiarthrasnán, the Irish word for diagonal.

Tarraing fiarthrasnán ar an gcearnóg.
Draw a diagonal on the square.
Roinneann an fiarthrasnán an dronuilleog ina dhá thriantán.
The diagonal divides the rectangle into two triangles.
Cén fad atá leis an bhfiarthrasnán?
How long is the diagonal?
Tá dhá fhiarthrasnán ag gach ceathairshleasán.
Every quadrilateral has two diagonals.
Trasnaíonn na fiarthrasnáin a chéile i lár an chrutha.
The diagonals cross each other in the middle of the shape.
Ríomh fad an fhiarthrasnáin, le do thoil.
Calculate the length of the diagonal, please.
Tá na fiarthrasnáin cothrom i ndronuilleog.
The diagonals are equal in a rectangle.

Other nouns in Irish

More everyday Irish nouns to learn next.

Diagonal in Irish: frequently asked questions

What is the Irish for diagonal?

The Irish for diagonal is fiarthrasnán (pronounced “FEER-hras-nawn”). It’s a masculine noun and the plural is fiarthrasnáin.

How do you pronounce fiarthrasnán in Irish?

Fiarthrasnán is said like “FEER-hras-nawn” (IPA /ˈfʲiəɾˌhɾasˠn̪ˠaːnˠ/).

What is the plural of diagonal in Irish?

The plural of fiarthrasnán is fiarthrasnáin; with the article, na fiarthrasnáin means “the diagonals”. The genitive plural is fiarthrasnán, as in líon na bhfiarthrasnán (number of diagonals).

Why does fiarthrasnán change to fhiarthrasnán or bhfiarthrasnán?

Irish changes the start of some words after certain words. After mo (my) fiarthrasnán becomes m'fhiarthrasnán (lenition), and after ár (our) or i (in) it becomes ár bhfiarthrasnán (eclipsis).

What is the difference between 'fiarthrasnán' and 'trasnán'?

'Fiarthrasnán' is the specific term for a diagonal in geometry, while 'trasnán' is a more general word for a crosspiece or transversal line.

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