A modular circuit matrix, also referred to as a skill matrix,[1] or Skill Module, was a swappable piece of droid programming. The matrices held only one skill at a time and were cheap and easy to install. The matrices could be used to modify a droid's knowledge and skill base. SE4 servant droids and MSE-6 series repair droids were equipped to use the matrices. For SE4 servant droids, the matrix could be slotted into a insertion slot on the unit's chest. These matrixes allowed the droid to absorb specific knowledge on subjects such as local cuisines and maintenance schedules designed to look after household estates.[2] MSE-6 droids were installed with a single matrix; each matrix was programmed with a single skill, such as the repair of a specific system,[1] janitorial cleanup, security protocols, and basic computer programming.[2] MSE-6s could perform one task, then have the circuit matrix switched out to then perform a different task.[1]