En este trabajo se exploran las premisas con que las que Lacan introduce su concepto de rasgo unario dentro del Seminario 9, La identificación. Durante la primera parte de este seminario es presentada una novedosa y radical tesis: la identificación no consiste sino en las relaciones del sujeto con el significante y sus efectos. Para sustentar tal tesis parte del término einziger Zug tomado de Freud y acuña su propio concepto de rasgo unario. Aunque la expresión ‘un único rasgo’ es usada por Lacan con anterioridad, es dentro del seminario dedicado a la identificación que el rasgo unario cobra un carácter preciso y decisivo en su enseñanza. Palabras clave Identificación Rasgo Unario Significante ABSTRACT THE INTRODUCTION OF THE UNARY TRAIT IN THE 9TH SEMINARY, THE IDENTIFICATION In this paper, the premises used by Lacan to introduce his concept of unary trait in the 9th Seminary: The Identification, are explored. During the first part of this seminary, a new and radical thesis is presented: identification is nothing more than the relations existing between the subject and the signifier. To sustain such thesis, he parts from the term einziger Zug, developed by Freud, and creates his own concept of unary trait. Even though the expression ‘a single trait’ is used by Lacan before, it gets a precise and decisive character for his teaching within the identification dedicated seminary. Key words Identification Unary Trait Signifier