mixins - is the dot necessary?

[jtousek]

_item (@bg: none) { background-image: @bg; } .item1 { _item(url("/images/home/item1_background.png")); }

I've tried to use something like this, but it cause a problem. Do the mixins with args need a dot on the begining? If yes, then why? It is not a class! Variables have @ but mixins nothing. And on the top, this is working:

_test{ font-size: 12em; } h2 { _test; }

Mixin with no dot on the begining but also no argument works fine, why that is not possible for mixins with aguments?