Open tunings are different to standard tuning as you tune your guitar into a chord either in major, suspended, minor, sharps or flats of the key that you want to tune to. Theres an open tuning for every chord like G,D,A etc. There are many other tunings you can use to tune your guitar in either lower pitch or higher pitch. Open tunings were invented many years ago and still is being used today. Open tunings were mostly used for slide playing for hawaiian guitar and also for the early style of blues the delta blues in the old days. Open tunings are also used for fingerpicking as well in mostly folk music, blues and many other styles.