Clarification on my last question

Question

Do all ordinal numbers ending in 1/3 undergo the change? I understand the first word will not (decimoprimero or vigésimo tercero will NOT be decimprimero or vigésim tercero),, BUT the second (primero/tercero) does??

Ex/ decimoprimer.

vigésimo tercer

Silvia Piriz · Accepted Answer

Hola Mia S.
Yes, you're absolutely right in your understanding. When it comes to compound ordinal numbers like decimoprimero or vigésimo tercero, only the second word—primero or tercero—undergoes the apócope (shortening) before a singular masculine noun.
That means we say el decimoprimer capítulo or el vigésimo tercer aniversario, not decimoprimero capítulo or vigésimo tercero aniversario. The first part of the compound (like decimo- or vigésimo) stays the same.
So in short: only primero becomes primer and tercero becomes tercer in these cases.
¡Buen fin de semana!
Silvia