17 CFR 300.105 - Joint accounts.
(a) A joint account shall be deemed to be a “qualifying joint account” if it is owned jointly, whether by the owners thereof as joint tenants with the right of survivorship, as tenants by the entirety or as tenants in common, or by husband and wife as community property, but only if each co-owner possesses authority to act with respect to the entire account.
(b) Subject to paragraph (c) of this rule, each qualifying joint account with a member shall be deemed held by one separate customer of the member.
(c) All qualifying joint accounts with a member owned by the same persons shall be deemed held by the same customer so that the maximum protection afforded to such accounts in the aggregate shall be the protection afforded to one separate customer of the member
(d) A joint account with a member which does not meet the requirements of paragraph (a) of this rule shall be deemed to be an individual or qualifying joint account of the co-owner or co-owners having the exclusive power to act with respect to it.