Recorded Texas Historic Landmark (RTHL) is a designation awarded by the Texas Historical Commission for historically and architecturally significant properties in the state of Texas. RTHL is a legal designation and the highest honor the state can bestow on a historic structure. Purchase and display of a historical marker is a required component of the RTHL designation process. Because it is a legal designation, owners of RTHL-designated structures must give 60 days notice before any alterations are made to the exterior of the structure. Changes that are unsympathetic may result in removal of the designation and historical marker. There are over 3600 RTHL structures throughout the state.