: When is a note flat/sharp? For instance we are playing a B, is the third note of the chord a Eb or and D#? If we play Bm is the fifth a Gb or a F#?

Try not to mix sharps and flats.Start a scale with whatever note. Count up, in order, T,T,S,T,T,T,S. T=tone, s=semitone. Use the next letter for the next note.

So, in C (no b or #) the order is C D E F G A B. Start on Eb. The order is Eb F G Ab Bb C D. The TTSTTTS is the same. The letter names are alphabetic. It works simply.

Now, to answer. Your B - third letter up - D. It's 2 tones up, so it must be D#. Any sort of E would be 4th.This is fairly straightforward elementary theory, but without it, nothing after will make any sense.

---

The interval quantity is always determined by counting the notes. In the key of A: A=1, B=2, C=3, etc.

So if you're in the key of B, all thirds will be some type of D, and all 5ths with be some type of F.

Once you have the interval size, then you determine its quality - sharp or flat, major/minor/perfect/augemented/diminished - depending on the root and the context.

So:

So, if for example, your tonic is B:

D# is the Major 3rd. (two whole tones from the root) Eb would be a Diminished 4th (one semi-tone below the Perfect 4th, E). D# and Eb are called enharmonic equivalents: Different spellings for the same note/pitch.

F# is the Perfect 5th.(3 1/2 whole tones from the root) Gb would be a Diminished 6th (one semi-tone below the Minor 6th, G) - again, enharmonic equivalents.

---

This is one of those questions that seems really obvious once you have a decent amount of experience, but can be a little challenging to explain.

I'm going to try and give you a couple of approaches that might help. All of the three ideas below are explaining the same thing from different starting points. Hopefully one of them makes sense.

After the first section, I've written a few quick comments on other connected ideas, or questions that might result from the other sections.

First idea. Are you familiar with your key signatures? Key signatures always have flats or sharps; never a combination of both. If our chord progression stays entirely within the key signature, we get an easy shortcut. If the key signature has flats, you're going to spell everything as a flat, not a sharp. If it has sharp, you're going to spell everything as a sharp.

Let's take your example. The key of B Major has five sharps. So, we're going to spell the accidentals as sharps, not flats. Which means the notes are D# and F#.

What about Cb Major? It has seven flats. Now, if you play this scale, it will sound the same as B Major (ignoring temperament, which is superfluous to this discussion). We're going to spell the accidentals as flats, not sharps. Which means the notes are Eb and Gb.

Second idea. Every major and minor scale has seven notes. Each of those notes has a name; it's a letter, and sometimes an accidental. Within the scale, we want to make sure that every note has a different letter. We don't want to be seeing Eb and E in the same scale; it makes things messy.

Back to your example. Following this single-letter rule, we can only spell it:

B C# D# E F# G# A# B

If we try to use Db instead of C#, we won't be able to complete the scale without repeating a letter somewhere, unless we spell the B as a Cb. But that's a Cb Major scale, not a B Major scale.

Third idea. Intervals. Each interval will have an associated letter. In a B scale:

The second will be some sort of C
The third will be some sort of D
The fourth will be some sort of E
The fifth will be some sort of F
The sixth will be some sort of G
The seventh will be some sort of A
The octave/unison will be some sort of B

So, the third must be a D#. It can't be an Eb, because that would be a fourth of some type (diminished, specifically). You can apply this pattern to any scale.

Notes and Chords. This all applies equally to both notes and chords of a scale. The third note of B Major is called D#. The third chord of B Major is called D#m (hopefully it's clear why it's a minor chord; if not, comment). The fifth of B Major is called F#. The fifth chord is called F# [major] (not Gb major).

On guitar chord charts. There's a tendency online for chord charts to refer to Bb as A#, and Eb as D#. That's ok, if the piece is in a key like C# Major or F# Major, because those are the correct spellings. It's not ok if the piece is in F Major or Bb Major or Eb Major. Here's some examples:

F Dm A# C (we're in F Major/D Minor, the key signature has one flat, so it should be F Dm Bb C)

Fm D#/F A# Fm (we're in F Minor/Ab Major, the key signature has four flats, so it should be Fm Eb/F Bb Fm)

On 'weird' keys. You may already know this, but once you get up around five, six and seven flats/sharps, you can pretty much choose between a flat key or a sharp key, and it will work either way. That's cool, and confusing.

The equivalences you're most likely to encounter are:

Five Flats == Seven Sharps (Db Major is the same as C# Major)
Six Flats == Six Sharps (Gb Major is the same as F# Major)
Seven Flats == Five Sharps (Cb Major is the same as B Major)

There are other equivalences, but let's not muddy the waters. You can also argue that these scales are not technically the same, but I think that's going way too far for this discussion.

How do you choose the right one? It's pretty much up to you, but you must be consistent. If you're going to call it C# Major, you can't use Ab chords. They are G#s. No mixing and matching.

Out of Key Signature Chords. Still with me? One last point. Sometimes a chord progression uses chords that aren't actually in the key signature. This means you need some extra accidentals. In this case, prefer flats if the key signature already has flats, and sharps in the opposite case. But this isn't as clear cut, and it depends on the chord progression. As you gain experience, you'll learn that people expect to see Bb chords in C Major, not A# chords. There are other theoretical reasons for these conventions, but this answer is far too long already, so I'm going to stop typing now.

---

You've answered the question by calling the intervals a 'third' and a 'fifth'. Whether perfect, major, minor, dimininished, augmented or anything else, a 'fifth' encompasses five letter names. If the root is some sort of B, the fifth will be some sort of F.

B, C, D, E, F.

1, 2, 3, 4, 5.

A fifth.

B, C, D.

1, 2, 3.

A third.

---