What does cream of tarter do in the RI recipe? In Antonia 74 recipe do I still need to add it in if the meringue powder has cream of tarter in it? Thanks for your help.

I believe cream of tartar helps to "stabilize" the icing.

Some meringue powder has it already in it (I think I remember Antonia74 saying hers does not), but I use a meringue power with cream of tartar and I add it anyway because I don't know how much cream of tartar is in the meringue powder....

Cream of tartar helps the egg whites to hold air. Most meringue powderes include it as an ingredient, along with the powdered egg whites. Only 1/8th tsp per egg white (from a large egg) is needed. Additional cream of tartar should not be needed with the meringue powder.