A slide, (neck of a bottle, knife blade or round metal or glass bar or cylinder) is used in blues and rock to create a glissando or "Hawaiian" effect. The slide is used to fret notes on the neck, instead of using the fretting hand's fingers. The characteristic use of the slide is to move up to the intended pitch by, as the name implies, sliding up the neck to the desired note. The necks of bottles were often used in blues and country music as improvised slides. Modern slides are constructed of glass, plastic, ceramic, chrome, brass or steel bars or cylinders, depending on the weight and tone desired (and the amount of money a guitarist can spend). An instrument that is played exclusively in this manner (using a metal bar) is called a steel guitar or pedal steel. Slide playing to this day is very popular in blues music and country music. Some slide players use a so-called Dobro guitar. Some performers who have become famous for playing slide are Robert Johnson, Elmore James, Ry Cooder, George Harrison, Bonnie Raitt, Derek Trucks, Warren Haynes, Duane Allman, Muddy Waters, Rory Gallagher, and George Thorogood.
For the standard tuning, there is exactly one interval of a major third between the second and third strings, and all the other intervals are fourths. The irregularity has a price - chords cannot be shifted around the fretboard in the standard tuning E-A-D-G-B-E, which requires four chord-shapes for the major chords. There are separate chord-forms for chords having their root note on the third, fourth, fifth, and sixth strings.
I would especially like to stress the gentle approach Justin takes with two key aspects that contributed to my development as a musician - music theory and ear training. Justin has succeeded in conveying the importance and profoundness of understanding music both theoretically and through your ears while maintaining a simple and accessible approach to them, all while sticking to what is ultimately the most important motto: 'If it sounds good, it is good'.