Thrash

Thrash Metal is a sub-genre of rock that focused on an aggressive, "underground" sound. This meant heavily distorted guitars, screamed vocals, and up-tempo songs with non-stop drumming. Guitars typically played riffs, rarely playing sustained chords. Riffs took inspiration from both blues and classical compositions. Fast guitar solos were also prominent. Rather than use rich harmonies for chorus sections, thrash typically built tension using rhythm drops, vocal-guitar point-counterpoint, or tempo shifts. Song structures often took experimental routes, sometimes omitting choruses altogether. Popular thrash artists are Metallica, Megadeth, Pantera, and Slayer.

Riff-based Tonality

Thrash artists rarely played even basic triad chords, instead focusing on riffs (outside of clean intros or interludes). A riff at its most basic level is simply a repeating single-note melody line. While rock music generally expanded upon this, thrash took it to the next level. Given that heavy distortion tends to compress the sound, using conventional dynamics by altering picking strength provided little tonal difference. Additionally, thrash's absurdly fast tempos made it difficult to apply contrasting picking strength during a riff. Seeking contrast for the notes inside riffs, artists expanded some notes as power chords while palm muting others.

Scale

Tonality mostly came from the scale the riffs were played in, which was typically a natural minor or a blues or a combination of the two. At times it may even become completely chromatic. Many songs revolve around the low open E, using an E minor or E blues scale (or Eb if tuned down a half step). It could also be or modulate to A (or Ab).

Pedal Points

A pedal point is a note that is held constant or consistently reinforced over a passage of changing harmonic content. For thrash metal, this typically meant low E or A string palm mutes being used as the backbone of a riff, with other notes playing off this tone.

d-------------9-----7-/9--------9-----7-/9--
a-------------7-----5-/7--------7-----5-/7--
e-0--0--0--0-----0--------0--0-----0--------
  p  p  p  p     p        p  p     p

d-------------5-----5-/4--------5-----5-/4--
a-------------3-----3-/2--------3-----3-/2--
e-0--0--0--0-----0--------0--0-----0--------
  p  p  p  p     p        p  p     p

Sometimes thrash artists would change the pedal point but continue playing the same riff otherwise, altering the feel of the riff.

d-------------9-----7-/9--------9-----7-/9--
a-0--0--0--0--7--0--5-/7--0--0--7--0--5-/7--
e-------------------------------------------
  p  p  p  p     p        p  p     p

d-------------5-----5-/4--------5-----5-/4--
a-0--0--0--0--3--0--3-/2--0--0--3--0--3-/2--
e-------------------------------------------
  p  p  p  p     p        p  p     p

Spider Fingering

With thrash riffs often playing consecutive power chords with different root notes, some players noticed they could use alternating index/ring and middle/pinky fingers to finger the chords, achieving greater economy of motion (allowing them to play even faster) and avoiding any scratchy sounds from sliding positions up and down between chords.

d-------4--3-----2--3-----
a-2--3--2--1--4--0--1--2--
e-0--1--------2--------0--
  m  r  p  r  p  m  r  m
  o  i  m  i  m  o  i  o

Black Notes (Metal's Blue Notes)

Blues used minor 3rds and 7ths and diminished 5ths for a distressed sound. Thrash similarly would substitute in minor 2nds and diminished 5ths into a minor scale to get an evil or doom-inspired sound. Rather than simply using these notes in passing, they would often be accented in riffs.

Shred

With thrash already concerned with speed and shredding guitar solos already popular in the Hair Metal scene, thrash had to take things even faster and flashier. Song structures were adjusted to incorporate more solos and/or with longer length.

Downpicked Palm Mutes

Although thrash was often focused on speed, many prefered to downpick all palm mutes to get them more of a repetitive and percussive sound. This didn't mean that tempos were lowered, but that large amounts of practice went into getting rhythmically-consistent, fast, consecutively-downpicked notes. This requirement was often relaxed for unmuted notes or subdivisions of the normal rhythm.

    d   d   d   d   d   d   d   d  |d u d   d   d   d   u   d   u   |
e --0---0---0---0---0---0---0---0--|0-0-0---0---0---6---5---3---5---|
    p   p   p   p   p   p   p   p  |p p p   p   p                   |