Exchanges, anticipations, perseverations, blends, and non-contextual substitution errors can be characterized by two units, which are referred to as the "interacting units". For exchanges, the two exchanging units are the interacting units; for anticipations and perseverations, the target, or intended unit, and the unit that moves to replace it are the two interacting units; and for substitutions, the target unit and the one replacing it are the interacting units.
- Dell (2004), Pag. 126