An engineer is a professional practitioner of engineering, concerned with applying scientific knowledge, mathematics and ingenuity to develop solutions for technical problems. Engineers design materials, structures and systems while considering the limitations imposed by practicality, safety and cost. The word engineer is derived from the Latin roots ingeniare ("to contrive, devise") and ingenium ("cleverness"). Engineers are grounded in applied sciences, and their work in research and development is distinct from the basic research focus of scientists. The work of engineers forms the link between scientific discoveries and their subsequent applications to human needs. A typical lineman's handset integrates an earpiece, a mouthpiece, a dialing interface, and a set of test leads for connecting to the telephone circuit. Originally, lineman's handsets featured a rotary dial, but modern sets use some variant of the standard 12-button DTMF keypad. Most handsets are designed to be used with analog "POTS" lines, and have limited or no function with digital circuits.