The chamber of secrets

Nationalities in French(masculine and feminine) 

1. The feminine form is usually constructed by adding an –e.
Ex. anglais + e = anglaise
2. If the masculine form ends in -en or -on, double the n and add an -e for the feminine form:
ex. macédonien+ne=macédonienne
3. If the masculine-singular form of an adjective ends in an -e, there is no difference between the masculine and feminine forms.
Ex. britannique=britannique
4. Exceptions
Ex. turc-turque
grec-grecque