#1
I've come across a few chord progressions that I like a lot but don't fully understand the theory of and I was wondering if someone could shed some light on the theoretical reasons why these progressions work.

The first is E G# C#m A, and I believe it's in the key of E major. What about the progression allows the III chord to work?

The other is:

D|-------------5
A|--3--5--7--3
E|--1--4--5----

I think it's in C major. Am I right in assuming that the 2nd chord is a D5 power chord, but with a flattened 5 as the bass note? What allows that to sound good?
#2
Quote by KireiNaMono
I've come across a few chord progressions that I like a lot but don't fully understand the theory of and I was wondering if someone could shed some light on the theoretical reasons why these progressions work.

The first is E G# C#m A, and I believe it's in the key of E major. What about the progression allows the III chord to work?


The III is often used as a "mediant" between the I and another chord (in this case, the C#m). It could be treated as an extension of the I chord. It will pretty much always sound good after the I. This is because they share two notes.

The other is:

D|-------------5
A|--3--5--7--3
E|--1--4--5----

I think it's in C major. Am I right in assuming that the 2nd chord is a D5 power chord, but with a flattened 5 as the bass note? What allows that to sound good?

I would say it's in A Minor.

The tritone acts as a leading tone to the A5. It's the nat. 7 that Harmonic Minor has.
#3
As to the first question, I - III7 - IV is a common progession, and C#m (C# E G#) is close to A (A C# E), and even closer to Amaj7 (A C# E G#).