Bonjour David,

Historically, ‘grand’ used to have the same spelling for feminine or masculine form. Through time, it got a feminine form too, except in compound words. Therefore, in feminine compound nouns, ‘grand’ agrees in number but not in gender:

singular: une grand-mère = a grandmother

             la grand-rue = the high street

Plural: des grands-mères = some grandmothers

          les grands-rues = the high streets

Attention: 

les arrière-grands-mères = the great-grandmothers

les arrière-grands-parents = the great-grandparents

 

I hope this is helpful.

Bonne journée!