15 U.S.C. § 1052 : US Code - Section 1052: Trademarks registrable on principal register; concurrent registration

      No trademark by which the goods of the applicant may be
    distinguished from the goods of others shall be refused
    registration on the principal register on account of its nature
    unless it - 
      (a) Consists of or comprises immoral, deceptive, or scandalous
    matter; or matter which may disparage or falsely suggest a
    connection with persons, living or dead, institutions, beliefs, or
    national symbols, or bring them into contempt, or disrepute; or a
    geographical indication which, when used on or in connection with
    wines or spirits, identifies a place other than the origin of the
    goods and is first used on or in connection with wines or spirits
    by the applicant on or after one year after the date on which the
    WTO Agreement (as defined in section 3501(9) of title 19) enters
    into force with respect to the United States.
      (b) Consists of or comprises the flag or coat of arms or other
    insignia of the United States, or of any State or municipality, or
    of any foreign nation, or any simulation thereof.
      (c) Consists of or comprises a name, portrait, or signature
    identifying a particular living individual except by his written
    consent, or the name, signature, or portrait of a deceased
    President of the United States during the life of his widow, if
    any, except by the written consent of the widow.
      (d) Consists of or comprises a mark which so resembles a mark
    registered in the Patent and Trademark Office, or a mark or trade
    name previously used in the United States by another and not
    abandoned, as to be likely, when used on or in connection with the
    goods of the applicant, to cause confusion, or to cause mistake, or
    to deceive: Provided, That if the Director determines that
    confusion, mistake, or deception is not likely to result from the
    continued use by more than one person of the same or similar marks
    under conditions and limitations as to the mode or place of use of
    the marks or the goods on or in connection with which such marks
    are used, concurrent registrations may be issued to such persons
    when they have become entitled to use such marks as a result of
    their concurrent lawful use in commerce prior to (1) the earliest
    of the filing dates of the applications pending or of any
    registration issued under this chapter; (2) July 5, 1947, in the
    case of registrations previously issued under the Act of March 3,
    1881, or February 20, 1905, and continuing in full force and effect
    on that date; or (3) July 5, 1947, in the case of applications
    filed under the Act of February 20, 1905, and registered after July
    5, 1947. Use prior to the filing date of any pending application or
    a registration shall not be required when the owner of such
    application or registration consents to the grant of a concurrent
    registration to the applicant. Concurrent registrations may also be
    issued by the Director when a court of competent jurisdiction has
    finally determined that more than one person is entitled to use the
    same or similar marks in commerce. In issuing concurrent
    registrations, the Director shall prescribe conditions and
    limitations as to the mode or place of use of the mark or the goods
    on or in connection with which such mark is registered to the
    respective persons.
      (e) Consists of a mark which (1) when used on or in connection
    with the goods of the applicant is merely descriptive or
    deceptively misdescriptive of them, (2) when used on or in
    connection with the goods of the applicant is primarily
    geographically descriptive of them, except as indications of
    regional origin may be registrable under section 1054 of this
    title, (3) when used on or in connection with the goods of the
    applicant is primarily geographically deceptively misdescriptive of
    them, (4) is primarily merely a surname, or (5) comprises any
    matter that, as a whole, is functional.
      (f) Except as expressly excluded in subsections (a), (b), (c),
    (d), (e)(3), and (e)(5) of this section, nothing in this chapter
    shall prevent the registration of a mark used by the applicant
    which has become distinctive of the applicant's goods in commerce.
    The Director may accept as prima facie evidence that the mark has
    become distinctive, as used on or in connection with the
    applicant's goods in commerce, proof of substantially exclusive and
    continuous use thereof as a mark by the applicant in commerce for
    the five years before the date on which the claim of
    distinctiveness is made. Nothing in this section shall prevent the
    registration of a mark which, when used on or in connection with
    the goods of the applicant, is primarily geographically deceptively
    misdescriptive of them, and which became distinctive of the
    applicant's goods in commerce before December 8, 1993.

    A mark which would be likely to cause dilution by blurring or
    dilution by tarnishment under section 1125(c) of this title, may be
    refused registration only pursuant to a proceeding brought under
    section 1063 of this title. A registration for a mark which would
    be likely to cause dilution by blurring or dilution by tarnishment
    under section 1125(c) of this title, may be canceled pursuant to a
    proceeding brought under either section 1064 of this title or
    section 1092 of this title.