Bonjour Tom,

Thank you for your excellent question!

In this example, in order to avoid repeating the word ‘communauté’ twice, we write it down once in the plural form: ‘communautés’. In this case, the adjectives, which would have been used with ‘communauté’ in the singular form, agree in gender but not in number.

Mes grands-mères maternelle et paternelle = my maternal and paternal grand-mothers

I hope this is helpful.

Bonne journée!