The improved leader block uses a substantially rectangular body that is equipped with two tape guide projections which form, in conjunction with the front of the body, a U-shaped channel open at one end and adapted to received the end of the magnetic tape that extends through the opening in the housing of the magnetic tape cartridge. The size and shape of this U-shaped channel substantially corresponds to the dimensions of the magnetic tape in order to precisely align the magnetic tape within the channel and with reference to the leader block body. The section of the front of the leader block body that constitutes the bottom of the U-shaped channel includes a plurality of parallel oriented, spaced apart grooves inscribed therein. These grooves extend from the top tape guide projection to the bottom tape guide projection in order to provide slots to receive corresponding projections that are included on the surface of the cover of the leader block. The cover fits into the U-shaped channel once the magnetic tape has been placed therein to sandwich the magnetic tape between the cover and the body of the leader block. The projections on the cover force the magnetic tape into the corresponding grooves in the face of the leader block body in order to thereby prevent the movement of the end of the magnetic tape with reference to the body of the leader block.