played by slanting the slide on certain notes. In the sixteenth century, the notes of A–D–G–B–E were adopted as a tuning for guitar-like instruments, and the low E was added later to make E–A–D–G–B–E as the standard guitar tuning. In open tuning the strings are tuned to sound a chord when not fretted, and is most often major. Open tunings commonly used with slide guitar include open D or Vestapol tuning: D–A–D–F–A–D; and open G or Spanish tuning: D–G–D–G–B–D. Open E and open A, formed by raising each of those tunings a whole tone, are also common. Other tunings are also used, in particular the drop D tuning (low E string tuned down to D) is used by many slide players. This tuning allows for power chords, which contain root, fifth and eighth (octave) notes in the bass strings and conventional tuning for the rest of the strings. Robert Johnson, whose playing has been cited by Clapton, Richards, Hendrix, and Winter as being a powerful influence on them, used tunings of standard, open G, open D, and drop D. Resonator guitars The National String Instrument Corporation produced the first metal-body resonator guitars in the late 1920s (see image at beginning of article). Popular with early slide players, these featured a large aluminum cone, resembling an inverted loudspeaker, attached under the instrument's bridge to increase its volume. It was patented in the late 1920s by the Dopyera brothers and became widely used on many types of guitars, and was adapted to the mandolin and ukulele. Tampa Red played a gold-plated National Tricone style 4, and was one of the first black musicians to record with it. Delta blues pioneer, Son House, played this type of guitar on many songs including the classic, "Death Letter". A resonator guitar with a metal body was played by Bukka White ("Parchman Farm Blues" and "Fixin' to Die Blues"). Lap slide guitar "Lap slide guitar" does not refer to a specific instrument, rather a style of playing blues or rock music with the guitar placed horizontally, a position historically known as Hawaiian style. This is a lap-steel guitar, but musicians in these genres prefer the term slide instead of steel; they sometimes play the style with a flat pick or with fingers instead of finger picks. There are various instruments specifically made (or adapted) to play in the horizontal position, including the following: a traditional guitar