Snobbish as it sounds and _is_, a brilliant ball is necessarily a collection of brilliantly fashionable people, and the hostess who gathers in all the oddly assorted frumps on the outskirts of society cannot expect to achieve a very distinguished result.
Ball invitations properly include all of the personal friends of the hostess no matter what their age, and all her better-known social acquaintances--meaning every one she would be likely to invite to a formal dinner. She does not usually invite a lady with whom she may work on a charitable committee, even though she may know her well, and like her. The question as to whether an outsider may be invited is not a matter of a hostess' own inclination so much as a question whether the "outsider" would be agreeable to all the "insiders" who are coming. If the co-worker is in everything a lady and a fitting ornament to society, the hostess might very possibly ask her.
If the ball to be given is for a débutante, all the débutantes whose mothers are on the "general visiting list" are asked as well as all young dancing men in these same families. In other words the children of all those whose names are on the general visiting list of a hostess are selected to receive invitations, but the parents on whose standing the daughters and sons are asked, are rarely invited.
A lady who has a débutante daughter, but who has not given any general parties for years--or ever, and whose daughter, having been away at boarding-school or abroad, has therefore very few acquaintances of her own, must necessarily in sending out invitations to a ball take the list of young girls and men from a friend or a member of her family. This of course could only be done by a hostess whose position is unquestioned, but having had no occasion to keep a young people's list, she has not the least idea who the young people of the moment are, and takes a short-cut as above. Otherwise she would send invitations to children of ten and spinsters of forty, trusting to their being of suitable age.
To take a family or intimate friend's list is also important to the unaccustomed hostess, because to leave out any of the younger set who "belong" in the groups which are included, is not the way to make a party a success. Those who don't find their friends go home, or stay and are bored, and the whole party sags in consequence. So that if a hostess knows the parents personally of, let us say, eighty per cent. of young society, she can quite properly include the twenty per cent. she does not know, so that the hundred per cent. can come together. In a small community it is rather cruel to leave out any of the young people whose friends are all invited. In a very great city on the other hand, an habitual hostess does not ask any to her house whom she does not know, but she can of course be as generous as she chooses in allowing young people to have invitations for friends.