Notas bibliográficas a pie de página.

Para escribir notas bibliográficas generalmente se utilizan fraces latinas condenzadas cuyo significado y forma de uso se confunde algunas veces. A continuación el significado y aplicación correcta de estas fraces.
Ibid. : whenever an identical reference is placed right after the concerned reference; the page number can be the same as the previous note or can be different (in which case, you would indicate the different page number);
Idem. : whenever an identical reference is placed directly after the concerned reference (the page number must also be the same). Some will prefer using the term "Ibid.".
Op.cit. : when a book reference has previously been stated as a footnote or an endnote, but not directly before;
Loc.cit. : when an article reference has previously been stated as a footnote or an endnote, but not directly before;
Mas información en Como escribir correctamente notas bibliográficas de pie de página.