The lesson says "Note that while the forms de hauteur, de longueur, de largeur can also be used (though less commonly), you can NEVER say de profond."
Surely it would make more sense to say "Note that while the forms de haut, de long, de large are more common than de hauteur, de longueur, de largeur, you can NEVER say de profond."
Thank you for your comment! You are absolutely correct ! It has now been amended accordingly.
Merci et bonne journée !
Hauteur, longueur, largeur are nouns feminine.
Profond or profonde are adjectives
However I believe that we could write "de profondeur" because profondeur is also a noun feminine.
This is my understanding -- hope it helps.
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