GDC Tofflemire Matrix Retainer (MRTU) The Universal (Tofflemire) matrix system is used in Class II restorations. The primary function of the matrix is to restore anatomic proximal contours and contact areas. The matrix retainer has four parts: Head – U-shaped, has three guides, or slots, for the position of the band Locking vise - the sliding body that holds the band Long knob – changes the diameter of the loop Short knob – locks the band in place within the sliding body