Traditionally, both competitor identification and the analysis of inter-firm rivalry have been approached from an industry perspective. After that, researchers or analysts have calculated aggregate indexes (such as CR or Herfindahl) that synthesize its structure. However, the availability of detailed information has allowed researchers to design new proposals that capture the interdependences arising among firms more precisely. In this paper, we use one of such proposals in order to describe the competitive situation in the Spanish banking sector. Specifically, we use one of the dimensions introduced by Chen (1996), market commonality, in order to identify the main rivals of every bank, savings bank or credit unions that operate in our country. Within this context, we pay special attention to the identification of the most important rivals for Banco Santander. [ABSTRACT FROM PUBLISHER]