A numbered or pseudonym account is opened not under your name but under a number or pseudonym. The bank always knows your identity but this information is held by only a few people within the bank. All others just see a number and can't know who is the account holder.

Numbered accounts present some distinct advantages when it comes to privacy. First, whenever you call your banker, you just mention your pseudonym or number, not your name. All statements are printed with a number - not your name. When you receive a transfer from another Swiss bank, the bank sending the funds will only see a number and they won't know who owns the account. When you make a transfer to another Swiss bank, the receiving bank will only see "the bank on the behalf of a customer". All these differences increase the level of privacy of your account. They do not, however, make it anonymous and the same Swiss bank secrecy laws apply.