Analysis, Music Theory

What’s the Difference Between A Key Signature & Accidental?

Written By : Andrew Siemon

Key signatures are made up of “sharps” and “flats” – these are called accidentals.  Sharps look like a hashtag (#) and flats look like a lowercase B (b).  There is one other symbol that can be used as an accidental – the natural sign. So how are these things different? And more importantly, what separates key signatures and accidentals?

The difference between a key signature and an accidental is that a key signature sits at the beginning of each line of music and applies to every note affected by those sharps or flats throughout the song.  An accidental sits to the left of the note it affects and only applies to that one measure. 

What’s The Primary Difference Between A Key Signature and an Accidental?

1) Accidentals Are Used Very Temporarily

Accidentals - What's the Difference Between a Key Signature and An Accidental
Like I’ve said in the image, these are accidentals. These triads are in the key of C Major. We know that because there are no sharps or flats at the beginning of the measure. Accidentals are put into the music when we want to communicate that a note outside of the key signature or scale must be played.

The main difference between a key signature and an accidental is that accidentals are used very temporarily. In other words, its effect is transitory. Usually, they’re used for a note that is not in the key signature that the song is written in. Or sometimes they’re used to push a melody in a surprising direction.  

My researcher reached out to the composer and vocal coach, Mary Feinsinger, who had the following thing to say:

“The key signature is based on what accidentals are in the scale of the piece.  It’s a convenience to players because it avoids having dozens of accidentals cluttering up the score. It’s a big inconvenience to beginning players.”

EAT Email Mary Feinsinger - What's the Difference Between A Key Signature & Accidental
You can see that email here.

If you’re a beginner to standard notation, (as a matter of fact, I’m not the greatest at reading it which I stated before in my guide to music theory), key signatures can be quite a nuisance as Mary pointed out. In that way, I would say that accidentals are less of a nuisance to me than key signatures.

The thing with a key signature is that they tell which notes are flat and sharp throughout the entire song, which is something that we’ll point out in the following section.

2) Key Signatures Determine Which Notes Are Sharp and Flat Throughout the Entire Piece

Key Signature - What's the Difference Between A Key Signature And An Accidental

When a song is created, it falls into a “key” which determines how many sharps or flats there are in that song.  A conventional composer will use that key signature so all the sharps and flats are assumed within the score. 

If the song changes key for a few measures, (and lots of them do), the songwriter can choose to change the key signature, and then change it back again a few measures later.  But this is time-consuming for the composer and confusing for the reader.

The easier – and better – choice is to use a few accidentals in the couple of measures where a new key is implied. 

A few accidentals will usually do the job faster and more clearly. This is something that I first learned when I took the Punkademic Music Theory Comprehensive Complete Course via their All-Access Pass.

Let’s say we have a song that’s in the key of A major.  That means there will be three sharps at the beginning of each line of music. 

The notes F, C, and G will be sharp throughout the entire song unless accidentals cancel them or the key signature changes. 

The circle of fifths is a great tool for telling you which keys have what accidentals, understanding them, the order of 5ths and 4ths, and also major/minor keys.

If the melody steps into the key of A minor for two measures, some notes will need to be changed to naturals instead of sharps.  Using natural signs will make this easy to do.  In the following measure, the notes will automatically become sharp again.

Other Differences Between A Key Signature and An Accidental

1) Accidentals Only Apply To The Exact Note 

C# versus C - What's the Difference Between A Key Signature and an Accidental
The first C is the one with the accidental so it’s the only one that’s affected. The C right beside it is left alone.

If a middle C has a sharp to the left of it, only that exact C will be sharpened.  A higher or lower C won’t be affected by the sharp. This goes for all accidentals.  

2) Key Signatures Apply Across All Octaves

B-Sharp Major - What's the Difference Between A Key Signature and An Accidental

A C-sharp that appears within a key signature applies to every C up and down the range of the instrument.  Also, in piano music, a key signature applies to both hands (by the way, you can learn music theory with any instrument which I argued elsewhere on the site).  

3) Some Accidentals Can’t Be Used In Key Signatures

Sharps vs Flats - What's the Difference Between Key Signatures and Accidentals .jpg
You’ll never see a bunch of sharps and flats together like they are when they’re grouped together in a key signature at the start of a measure.

Key signatures consist of between one and seven sharps or flats.  Sharps and flats can’t be mixed. The natural symbol, double sharps, and double flats – which we’ll talk about in a minute – can’t be used in a key signature. At least not that I’m aware of.

4) It’s Possible To Mix Accidentals Unlike Key Signatures  

While sharps and flats are never used together in a key signature, there is a situation where accidentals can be used together.  If a note has been double-sharped and needs to be single-sharped, you’ll see a natural sign (to cancel the double sharp) followed by a sharp. 

Additionally, you can see some chords like the G-Minor/Major 7 chord that has a B-flat as well as an F#. This isn’t very common, but it’s possible to see the intermingling of flats and sharps within one piece. But you won’t even see the mix of sharps and flats at the start of the staff to indicate a key. It just isn’t a thing.

What Is An Accidental Key? 

Piano Keys - What's the Difference Between A Key Signature and An Accidental

An accidental key is any black key on the piano or a key that’s used to play a sharp, flat, double-flat, or double sharp.  In other words, when you see a sharp or a flat in front of a note in a piece of music, it means you will almost certainly play one of those black notes. 

There are some cases, however, where a white key could also be an accidental. As far as I know, though, it’s not super common to see white notes as an accidental.

1) Accidentals Raise Or Lower A Note By A Half-Step (Also Called A Semi-Tone)

A sharp raises the written note by a half-step.  In other words, on the piano, it pushes the tone up to the nearest black note.  A flat pushes the written note down to the nearest black note.  Sharpen up, flatten down. 

2) Notes With Accidentals Go By Different Names, ie, F-Sharp or B-Flat

Each note that has been changed by an accidental is renamed to include the accidental.  For instance, an F that has a sharp to its left is then called an F-sharp.  A G that has a flat beside it is called G flat, and so on.  

3) Accidentals Can Be White Notes Too (on the Piano)

As I was saying earlier about this, there are exceptions where white keys can represent an accidental.  There is such a thing as a C flat and an F flat.  Because there are no black notes to the left of the note C or F on piano, the lowered tones become B and E respectively.  The same applies to B sharp and E sharp. 

4) There Are More Advanced Accidentals Like Double Flats and Double Sharps

Double Sharp and Double Flat - What's the Difference Between A Key Signature & Accidental
The weird ‘X’ symbol amongst the #’s is what represents the double sharp. The double-flat, on the other hand, is literally just two flats.

There are two other accidentals to learn about – usually at an intermediate level.  These are double sharps and double flats.  These raise or lower the tone of the written note by two half-steps – or one whole step.  On a piano, this means the new tone is a white note.  

How Long Does the Effect Of An Accidental Last? 

An accidental lasts for the rest of the measure it appears in. This means that if that same note crops up again within the same measure, it’s still affected by the accidental unless there’s a different accidental in front of it, such as a natural.  

Once you’ve passed through the measure with the accidental, the music reverts to whatever the key signature originally indicated. An accidental applies to a note that is tied to another note that’s the same, both within a measure and when it’s tied across measures. 

Mark Sarnecki's Complete Elementary Rudiments - Why Do Some Pianos Start With A C and Not an A [ANSWERED]
The two books – Mark Sarnecki’s Complete Elementary Music Rudiments (on Amazon) and the Answer Book – I’m holding here are my favorites and they’re what I used to pass my music theory exam years ago. I recommend them to everyone who’s serious about learning.

Gear Mentioned

1) Mark Sarnecki’s Complete Elementary Music Rudiments and the Answer Book

2) Punkademic’s Music Theory Comprehensive Complete (on their site)

3) Punkademic’s All-Access Pass (on their site)

Andrew Siemon is the principal creator of, a website dedicated to all things music, including music production, music theory, recording, and how to use the most popular DAWs. Starting out as a metal guitarist, Andrew has since moved into other areas of music production including hip-hop and fusion

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