It's 'Ces Filles' in French, Not 'Cettes'

Although the singular is 'cette,' the plural is not 'cettes.'

Mistakes will always be made in French, and now you can learn from them.

Simply adding an s to the singular feminine cette to make the plural is not the way French has evolved. Cettes would be a big mistake. The correct plural in both masculine and feminine forms is ces, and that's just the way it is. Language isn't always logical.

Demonstrative Adjectives

Ce, cet, cette and ces are what the French call demonstrative adjectives.

Just as there is only one plural definite article for both masculine and feminine (les garçons, les filles) and only one plural possessive adjective (mes garçons, mes filles), there's only one plural demonstrative adjective: ces garçons, ces filles:

English Masculine Masc before vowel Feminine 
this, that   ce   cet   cette
these, those    ces   ces

   ces

Demonstrative adjectives are words used in place of articles (un, une, le, la, les) that point to a specific noun. In French, they must agree in gender and number with the noun they modify:

Ce is the masculine singular:
   Ce prof parle trop. > This (That) teacher talks too much.

Ce becomes cet in front of a masculine noun that begins with a vowel or a mute h, for ease of pronunciation:
   Cet homme est sympa. > This (That) man is nice.

Cette is feminine singular:
   Cette idee est excellente. > This (That) idea is excellent.

Ces is plural for both masculine and feminine nouns:
   Ces livres sont stupides. > 
These (Those) books are stupid.

Ces, again, is the only plural demonstrative adjective: Cettes does not exist. Do not use it, because that would be a sizable error.

How Do Demonstrative Adjectives Differ From Demonstrative Pronouns?

Demonstrative adjectives take the place of articles and point to a specific noun. If you're talking about a book you highly recommend, for instance, it's not just a book, but this particular book.

Demonstrative pronouns take the place of nouns that were previously mentioned. Imagine having to repeat a noun over and over again when you're speaking or writing; that would make the words bulky and boring. But mixing things up by replacing the nouns with demonstrative pronouns from time to time, avoids a lot of repetition and lightens things up. 

Demonstrative pronouns—this (one), that (one), the one(s), these, those—like demonstrative adjectives, must agree with the noun(s) they replace in gender and number: celui (masculine singular), celle (feminine singular), ceux (masculine plural) and celles (feminine plural).

The singular demonstrative adjectives ce, cet, and cette can all mean "this" or "that." Your listener can usually tell which you mean by the context. If you want to stress one or the other, you can use the suffixes -ci (here) and -là (there):

   Ce prof-ci parle trop. > This teacher talks too much.
   Ce prof-là est sympa. > That teacher is nice.
   Cet étudiant-ci comprend. > This student understands.
   Cette fille-là est perdue. > That girl is lost.

Ces can mean "these" or "those." Remember to use the suffixes when you want to be more explicit:

   Je veux regarder ces livres-là / ces livres-ci. > I want to look at those / these books.

Keep in mind that the demonstrative adjective ce never contracts. But for ease of pronunciation, it does change; in front of a vowel, ce becomes cet. (Note that the c' in the expression c'est is not a demonstrative adjective but an indefinite demonstrative pronoun).

Additional Resources

Definite articles
Demonstrative adjectives
Possessive adjectives
Demonstrative pronouns
Ces vs Se

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Lawless, Laura K. "It's 'Ces Filles' in French, Not 'Cettes'." ThoughtCo, Apr. 5, 2017, thoughtco.com/cettes-filles-french-mistake-1369451. Lawless, Laura K. (2017, April 5). It's 'Ces Filles' in French, Not 'Cettes'. Retrieved from https://www.thoughtco.com/cettes-filles-french-mistake-1369451 Lawless, Laura K. "It's 'Ces Filles' in French, Not 'Cettes'." ThoughtCo. https://www.thoughtco.com/cettes-filles-french-mistake-1369451 (accessed September 21, 2017).