Hacker News

Addressing HN comments:

Seems Just like the letter -y at the end of a or Spanish (or English?) word is a glorified i, to make it more legible // Ley, rey, etc.

As the other user correctly highlighted, Spanish didn't inherit Latin's nominative forms (lex, rex, nox...); it inherited the accusative ones (legem→ley, regem→rey, noctem→noche).

The pronunciation of those words changed. And alongside it, the spelling. That Latin ⟨g⟩ in legem, regem used to be pronounced [g], like in English "gift" or "good"; but eventually it was softened to a [j], as in English "yes". And the ending of the word was lost.

This wasn't just Spanish, mind you, but pretty much all Western Romance languages. (Italian sometimes dropped it - cue to re, but legge). Then how you write that /j/ down is up to spelling conventions: Spanish picked ⟨y⟩, Galician ⟨i⟩ (rei, lei), and French went back and forth (in MF the spelling was still "roy, loy", but nowadays "roi, loi").

Edit: on the other hand, x used to be read /j/ (e.g. Quixote, Mexico), so also possibly lex —> lej —> ley

Spanish ⟨x⟩ used to be read /ʃ/, as in modern French and Portuguese. (It's the first sound of "shampoo"). While ⟨j⟩ was used for the voiced counterpart, /ʒ/ (as in "Asia") or /dʒ/ (as in "jeans"). Eventually both merged, got backed to the modern /x/ (as Scottish "loch"), and the resulting merge is now spelled with ⟨j⟩.