Many builders are not musicians and have limited musical skills. It is often to difficult to know exactly how to tune strings on various instruments. MGB is not a musician and created his own cheat sheet to help him with set ups. While many musicians can get pretty close with tuning a string by ear, others (like MGB) have no idea where they are on the scale. MGB quickly realized it was helpful to use the corelating frequency for tuning. The A note has  frequency of 440HZ. The Pano tuner app is very helpful, it provides the note and the frequency. The 88 keys on the piano are listed on the left, moving to the right are popular note tuning for a number of instruments.  While there are many tuning possibilities for cbgs listed are a few popular choices.