This paper provides a history of robotic guitars and bass guitars as well as adiscussion of the design, construction, and evaluation of two new roboticinstruments. Throughout the paper, a focus is made on different techniques toextend the expressivity of robotic guitars. Swivel and MechBass, two newrobots, are built and discussed. Construction techniques of likely interest toother musical roboticists are included. These robots use a variety oftechniques, both new and inspired by prior work, to afford composers andperformers with the ability to precisely control pitch and plucking parameters.Both new robots are evaluated to test their precision, repeatability, andspeed. The paper closes with a discussion of the compositional and performativeimplications of such levels of control, and how it might affect humans who wishto interface with the systems.