Primer/tercer: decimoprimer? Decimotercer?

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:

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:

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

Hola J. S.

Thanks for pointing this out — I see how my wording could have caused some confusion. Let me clarify.

The apócope rule only applies to primero and tercero when they come right before a singular masculine noun. That’s why they shorten to primer and tercer:

• el primer día

• el tercer piso

This rule does not apply to other ordinal numbers that end in 1 or 3, whether they appear alone or as part of a compound number. So forms like undécimo, decimotercero, vigésimo primero, vigésimo tercer all stay the same.

So the takeaway is: only primero → primer and tercero → tercer change before a singular masculine noun. All other ordinals, even if they end in 1 or 3, remain unchanged.

Hope that clears it up!

Saludos
Silvia