Learn French: J'ajuste le tiers de la largeur. - I adjust a third of the width.

While you can absolutely say un tiers, French often prefers using the definite article le with fractions (le tiers, le quart, la moitié) when referring to a portion of a specific, known whole. Because it is a specific width being adjusted here, le tiers sounds very natural.

Why is the word for "third" tiers here instead of troisième?

Troisième is an ordinal number used to describe a position in a sequence, like "the third door" (la troisième porte). When talking about pieces or mathematical fractions, French uses specific fractional nouns. Halves, thirds, and quarters have special unique words: la moitié, le tiers, and le quart.

Is there a rule to help remember the gender of largeur?

Yes! Nouns expressing a dimension or physical quality that end in -eur are generally feminine. That is why we use de la before it. You will see this exact same pattern with other spatial measurements like la longueur (length), la hauteur (height), and la profondeur (depth).