A cutting insert has upper and lower faces, and a peripheral surface with two longitudinal insert side surfaces. Two similar through-going transverse insert key recesses open out to the insert side surfaces and to the insert lower surface. Each insert key recess has a similar profile when viewed in transverse cross sections of the cutting insert. Each insert key recess is limited by an insert recess top surface and insert recess side surfaces. The insert recess side surfaces are symmetrical about a plane. A holder blade for holding the cutting insert has upper and lower jaws for clamping the cutting insert therein, and a holder recess. The lower jaw has a leading surface and lower jaw recess. A cutting-tool key has two key members, fitted into the holder recess and an insert key recess, to force the cutting insert to mount it to or extract it from the cutting tool.