Why is the ending of Mediolanensis in Archiepiscopus Mediolanensis and Archidioecesis Mediolanensis the same even though the former noun is masculine and the latter feminine?
There are two (main) classes of adjectives in Latin:
- Some adjectives use the first declension for feminines (e.g. Romana, "Roman") and the second declension for masculines and neuters (e.g. Romanus and Romanum). For adjectives like this masculine forms look different from feminine forms — with the exception of plural dative and ablative.
- Some adjectives use the third declension for all genders (e.g. Mediolanensis/Mediolanensis/Mediolanense, "Milanese", for feminine/masculine/neuter). For these adjectives masculine and feminine forms look almost always the same. The only exception can be in the singular nominative, but only for some adjectives (e.g. celeris/celer/celere, "fast", for feminine/masculine/neuter). For Mediolanensis there is no difference between feminine and masculine.
In short: Some adjectives look different in masculine and feminine, some don't.
This is also true in Romance languages. In Italian you have the two forms romana and romano ("Roman") for the two genders (Italian has no neuter) but milanese ("Milanese") is both feminine and masculine.
To supplement Joonas's excellent answer, Latin has five classes of adjectives, which are generally put into two main groups.
- "First/Second Adjectives"
- "Normal" adjectives: these take the standard endings for the first and second declensions, and look different in all three genders. Bonus is one of these: bonus, bona, bonum.
- "Irregular" adjectives: these are the same as the normal ones, except that the masculine genitive and dative singular use the older endings -ius and -ī instead of -ī and -ō. There are only nine of them in the entire language, which are commonly learned with the mnemonic UNUS NAUTA.
- "Third Adjectives": these ones decline like third-declension nouns.
- One termination: these adjectives look the same in all three genders, like atrox, atrox, atrox. The only gender differences are the standard rules for the neuter (-ia in the plural, accusative matches the nominative). Present participles are in this class.
- Two terminations: these are the most common "third" adjectives. They have one form in the masculine and feminine, and a different form in the neuter, like omnis, omnis, omne. Comparative adjectives also tend to be in this class, like melior, melior, melius.
- Three terminations: these are the rarest ones. They have different forms in each different gender, and the masculine always ends in r, such as celer, celeris, celere.
In this case, you've found a third two-termination adjective, so the masculine and feminine look the same.