 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.