I'm writing a little song, It seems to me as the key is in Em as it has that 'home' feel and most of the chords I'm using correspond.
However, my lead asked me what key what we were playing in so he could work on some riffs and I when I looked it up to be sure, I ended up less sure.

The chords I'm using are Em, A#(that's the one throwing me the most) B, F and F# Ofcourse, I've considered that I'm borrowing outside sounds, I'm playing thrash and maybe it's just my ear for the genre and familiar riffs but no sound seems 'outside' to me, but on closer inspection when I just play the chords plain it seems that maybe I am 'borrowing' the A#

Anyone fancy helping me understand what I'm doing here?

much appreciated!
First and foremost: can you name the notes that fit into the Em scale? You must always be sure you know the notes that go inside. The key of Em has the notes: E F# G A B C D
So you have borrowed the F note and also the A#. Now if you say you play thrash, I'll assume the bulk of the song is riffage and powerchords. Now, the A# isn't something to be worried about, the A# is an augmented fourth of E, which is part of the E blues scale, which is used a lot in rock and metal. (the augmented fourth is usually thrown into the minor scale and used all together)
The F is a little different, the chord sequence you provided suggests that after F and F#, it goes back to E, which sounds uneasy because they are three adjacent chromatic notes. While some classical composer would cringe at ya, you're writing metal, and metal has quite a lot of "borrowed" notes added for flavour, especially the ones that form dissonant intervals. I don't know how your song will go, but the progression definitely fits the genre.

Listen to Master of Puppets (the song), it has A# chords in the verse and F chords in the end of the chorus... and it works. So don't worry about the borrowing, but be aware of which notes you are using that do fit the key, if you end up with too much borrowed notes you might be staring down a chromatic mash-up big mess.
The trick is not to linger on the borrowed notes too long or it'll throw the listener (and the lead guitarist) off, and not in a good way. You want to use it to add extra flavour and/or suspense between two chords that fit into the key completely, so you transition to the second chord quickly. Good luck with your song!
Last edited by Navi_96 at Jan 9, 2014,
Thanks very much for the excellent advice. I do just about know the Em key scale and I knew the F didn't fit in but the A# seems to sound more foreign, maybe it's because the F is used mostly in conjunction with an Em

The F is perhaps more integral to the riffs, but the A# is an important one aswell, it gives the rifffs that sharp 'bark'. I gave the chords in order of what is played first, the actual chord progression would be: Em, A#, F#, A#/B/A#. Em, A#, F#. (I have also experimented with replacing the final F# with a F)

You are correct that the song mostly utilises riffs and power chords, another, similar riff goes, Em, F, Em, A#, would this riff be 'legal' and fit in the same key?

and I'm aware that thrash has a tendency to use notes chromatically, which I guess lets me off a little, and now that you mention MOP (luckily I song I can play so I can visualise what you're talking about) I can't recall the A# (but plenty of B) For reference I assume that MOP is in the key of Em? (it seems to be as I've worked it out, all the chords are there)

But the advice has been helpful, especially from someone who understands the genre! As I said, much appreciated!
No problem mate, I'm always there to help!
Concerning the E F E A# riff, it's perfectly legal... though since the F is borrowed and the A# is included via the blues scale, the riff itself doesn't quite fit into the Em key, it implies more of a D minor scale (the E, F and A# being the 2nd, 3rd and 6th respectively), but since it is part of a whole, you'll easily connect it with the rest of the song, so the Em is safe.

And you're right, MoP is in E minor... Mettalica does a lot of stuff in Em, so the chugging at the low E string sounds right there at home . The A# appears in the verse where you first play the open E string 9 times, then G5, slide up to A5, then A#5, A5, G5, A5.
I guess it just threw me because 2/5 of the chords I was using so far didn't belong in the key, but thanks for clearing it up, I'll have a little look at the D minor and use that for some experimenting, for educational effect if nothing else, thanks.

and Ofcourse! Sorry, I was too focused on my A# on the second string to consider the first string, bit of a D'oh moment.
I wouldn't worry about the D minor scale. Your progression has an i-V cadence and you also feel that the "home" is E minor. I'm 99% sure it's in E minor. Use accidentals to accomodate for clashes over the out-of-key chords.
And no, Guitar Hero will not help. Even on expert. Really.