Half-Step Detuning by Matthias Young will teach you how to tune your guitar down a half-step from a standard tuning. You will be changing the strings’ pitch from (E, A, D, G, B, E) to (Eb, Ab, Db, Gb, Bb, Eb). However, most digital tuners don’t show pitches in terms of flats. So you will actually tune the strings to (D#, G#, C#, F#, A#, D#) according to your tuner’s display. You will end up with a more precise tuning if you tune down below the correct pitch and then tune the string up to the correct pitch. You can watch the video for more explanation.
Tuning down a 1/2 step can be useful for many reasons. It can switch a song to a more desirable key for a singer. It also can change the tension of the strings and thus the tone of the guitar. As a result, you may find that you will need to tune down to be able to play along with the original recording of a song. For example, you can learn how to play the opening riff from Sweet Child O’ Mine as it was played on the album.