Early medieval music finds its origins in the development of portions of scripture set to Roman melodies under the auspice of Pope Gregory I. These came to be known as Gregorian chant. They were monophonic in nature, carrying but one tone through the entire melody. It was during the late dark ages, approximately 900 to 1100 C.E., when a second melody parallel to the first was added to the chants, creating the original western polyphony. This was a primitive form of polyphony in that the parallel melodies duplicated the rhythm and pitch of the original melodies. This did, however, begin to establish the basic chord structures of future western music.
In the late medieval period, polyphony began to mature, with the polyphonic lines in music acquiring independence in pitch and rhythm from the basic melody. This was also expanded to more than one polyphonic line at a time, and necessitated the development of staff notation, the common way of transcribing music used to this day. During the Renaissance period, polyphonic composition was further developed by reducing the complexity of the polyphonic intricacy and establishing a clearer melody, making the music more accessible. This gave birth to modern forms of music with a primary melody overlaying the harmony of a song.
Heavy metal developed primarily from American blues, following similar chord progressions and the feature of the virtuoso guitarist. Bands such as Led Zeppelin and Black Sabbath, considered by-and-large the grandfathers of heavy metal, were blues-focused in the basic structure of their songs. As heavy metal became more established, bands began to experiment with moving away from blues-based structure and progression, moving into establishing complex polyphony in the interplay between melody, harmony and sub-harmony. This was exemplified by bands such as Iron Maiden and Judas Priest. In some instances these bands were even termed "neoclassical" due to their focus on late medieval chord structures and progressions.
One of the most defining features of modern heavy metal is quite possibly the use of arpeggio. Arpeggios are scales woven together and played at a rapid pace, with various repeats and skips in the scale to provide a rapid, fluttering musical narrative. Arpeggios were a late medieval development best exemplified by composers such as Vivaldi or Stravinsky who featured them heavily.
With heavy metal adopting the idea of the virtuoso guitarist from its blues roots, the arpeggio came to be front and center within the genre. Electric guitars were the ideal medium to create from them entire features of songs, creating an entire niche for guitar solos and soloists.
Where early heavy metal featured a blues modality, employing slow progression within their harmonies and largely flat keys, later bands began to employ a wider structure, employing neoclassical and medieval influences such as open or inverse chords, natural scales and counter-harmonies. The genre itself began to diverge with many bands sticking to the blues roots while others began to explore modalities from other cultural and musical periods. The medieval period began to find greater ground as bands began using basic polyphonic modes, which were then combined and overlaid against each other. Bands such as Helloween and Dream Theater pushed this concept, birthing genres such as progressive metal and power metal, replete with the late medieval concepts of full polyphony and arpeggios.