modern yet historically evocative sound. In an acid rock band, a guitar tech might have to manipulate the controls on a ring modulator or a rotating Leslie speaker cabinet to create unusual sounds while the guitarist is performing. Once the guitars have been tuned with an electronic tuner and strummed to ensure that they are in tune, the guitar tech usually sets up the different guitars on guitar racks, ensures that the leather or nylon straps are properly connected, and that the patch cords are plugged in properly. During the show, the guitar tech hands instruments to the guitarist or guitarists according to the types of guitar that are required in the songs that they are playing. For example, a hard rock guitarist may use a "flying-V" guitar for a fast song, and then switch to an acoustic 12-string guitar for a soft ballad. The guitar tech retunes all of the instruments before they are used, because even if an instrument was perfectly in tune during the soundcheck, the heat from stage lights and the humidity from the stage conditions may render the instrument slightly out of tune. After each guitar is used, the guitar tech cleans the strings with a cloth and replaces the instrument on a rack. During the show, the tech stands ready to replace any guitars in case a string breaks or if there is an equipment malfunction. The guitar tech may hand fresh towels to the guitarist so that the guitarist can remove sweat from the hands and ensure that the guitarist has ready access to bottles of cool water or other beverages. If a guitar technician is working for a guitarist who uses picks, the guitar tech may lay out a variety of picks on a guitar amplifier or tape the picks to the mic stands with double-sided tape, so that they are within easy reach. At the end of the show, the guitar tech disconnects all of the patch cords, cleans the instruments and puts them back into their cases. Maintenance and repair The guitar tech also might perform any of a variety of maintenance tasks, such as checking that the string height of the guitars is set properly, modifying ("dressing") the height and arc of the frets, adjusting the intonation of the instruments, checking that tubes (valves) on tube amplifiers are working properly, and that cables are in good condition and free