Primer/tercer: decimoprimer? Decimotercer?

Question

Do all numbers ending in 1 and 3 (11, 13, 21, 23, 31, 33) change when before masc sing nouns?  Or just for 1 and 3

Silvia Piriz · Accepted Answer

Hola Mia S.
Great question! In Spanish, only “primero” and “tercero” drop the final -o before a singular masculine noun, becoming “primer” and “tercer”.
Examples:
El primer día (not el primero día)
El tercer piso (not el tercero piso)
This apócope rule does not apply to other ordinal numbers ending in 1 or 3, such as undécimo, decimotercero, vigésimo primero, etc. Those stay the same:
El decimoprimer aniversario
El vigésimo tercer capítulo
So to answer your question: it’s only “primero” and “tercero” that change—regardless of whether they are used alone or as part of a compound number like vigésimo tercer.
Hope that helps!
Saludos
Silvia