Ciamar a sgrìobhas mi litir / How do I write a letter?

[Níall Beag]

Although female names only change for the first name:
[because female names don't slenderise in the vocative]

Catrìona NicAoidh > a Chatrìona NicAoidh
Màiri Mhoireasdan > a Mhàiri Mhoireasdan

Do you really mean only the first name? Isn't it everything but the last (genitive) name?

ie Catrìona Màiri NicAoidh -> Chatrìona Mhàiri NicAoidh

[ignore me -- see Stìophan's post below.]

[GunChleoc]

in the case of female names the vocative and the genitive are identical, so there is no change of form.

That makes sense to me.

[Stìophan]

Although female names only change for the first name:
[because female names don't slenderise in the vocative]

Catrìona NicAoidh > a Chatrìona NicAoidh
Màiri Mhoireasdan > a Mhàiri Mhoireasdan

Do you really mean only the first name? Isn't it everything but the last (genitive) name?

ie Catrìona Màiri NicAoidh -> Chatrìona Mhàiri NicAoidh

In which case the real rule would be:
All names go into the vocative.
BUT
in the case of female names the vocative and the genitive are identical, so there is no change of form.

Not True

Feminine Vocative = + Lenition
Feminine Genitive = + Slenderisation + final -e

What I meant was you would not notice if a feminine surname was in the vocative as they are either already lenited or begin with a vowel or l, n, r which can't be lenited.

All names in the should be put in the vocative where possible.