The [conjoint tendon] (sometimes called the conjoined tendon] is the common tendinous attachment of the internal oblique muscle and the transverse abdminis muscle into the pubic tubercle. Some of these tendinous attachments extend also to the pectineal (Cooper's) ligament, the inguinal ligament, and the superior ramus of the pubic bone. In the classical anatomical description these tendons mix to the point that they cannot be separated one from the other, hence the term [conjoint tendon].
The conjoint tendon is important in open hernia repair, where some surgical techniques require the surgeon to pass a surgical needle and suture through this tendinous structure, to attach or close the gap between the conjoint tendon and the inguinal ligament.
In spite of being described in many anatomy books, a true "conjoint tendon" is only found in about 4% of the cases (varying from 3 to 6%, depending on the author). What is usually found are slightly tendinous discrete structures that attach to the pubic tubercle. Because of this, Skandalakis, et al proposed to change the name to the "conjoined area".
Sources:
1. "Le Tendon Conjoint: Memoire realise dans le cadre du certificat d'anatomie, d'imagerie et de morphogenese" Leroux, H. Universite D' Nantes, 2005
2. "Hernia; Surgical Anatomy and Techniques" Skandalakis, J. et al. 1989 McGraw Hill
3. "Tratado de Anatomia Humana" Testut et Latarjet 8 Ed. 1931 Salvat Editores, Spain