EXHIBIT 5.1
                                 April 25, 2005

Board of Directors
First Horizon National Corporation
165 Madison Avenue
Memphis, TN 38103

Ladies and Gentlemen:

     I have acted as counsel to First Horizon National Corporation,  a Tennessee
corporation  (the  "Company"),  in connection with the  registration on Form S-8
(the "Registration Statement") under the Securities Act of 1933, as amended (the
"Act"), of deferred compensation  obligations of the Company (the "Obligations")
to be offered and sold pursuant to the terms of the First  Horizon  Nonqualified
Deferred  Compensation Plan (the "Plan").  I have examined  originals or copies,
certified or otherwise identified to my satisfaction, of such corporate records,
certificates  and  other  documents,  and  such  questions  of  law,  as I  have
considered necessary or appropriate for the purposes of this opinion.

     Upon the basis of such examination and subject to the limitations contained
herein,  it is my opinion that the Plan has been duly adopted by the Company and
that the Obligations, when issued in accordance with the terms of and subject to
the conditions of the Plan,  will be duly authorized and validly issued and will
constitute the valid and legally binding obligations of the Company, enforceable
against  the Company in  accordance  with their terms and the terms of the Plan,
except as enforcement  may be limited by (i) applicable  bankruptcy,  insolvency
(including,  without  limitation,  all laws relating to  fraudulent  transfers),
moratorium,  reorganization  or other similar laws affecting  creditors'  rights
generally  or (ii)  general  principles  of equity,  regardless  of whether such
enforceability is considered in a proceeding in equity or at law.

     The  foregoing  opinion is limited to the federal laws of the United States
and the laws of the State of  Tennessee,  and I  express  no  opinion  as to the
effect of the laws of any other jurisdiction.

     In rendering the foregoing opinion, I have relied as to certain matters, to
the extent I deem such reliance appropriate, on statements,  representations and
other information  obtained from public  officials,  officers of the Company and
other sources believed by me to be responsible.

     I hereby  consent  to the  filing  of this  opinion  as an  exhibit  to the
Registration Statement and to the reference to me in the Registration Statement.
In giving  such  consent,  I do not thereby  admit that I am in the  category of
persons whose consent is required under Section 7 of the Act.

                                 Very truly yours,

                                 /s/ Clyde A. Billings, Jr.
                                 -------------------------
                                     Clyde A. Billings, Jr.
                                     Senior Vice President, Assistant General
                                       Counsel and Corporate Secretary