The short answer

Different instruments are in different keys due to historical design constraints, the ease of switching between instruments of various sizes, and to simplify sheet music reading for musicians.

The long answer

You might assume that when different instruments play a note written as C on sheet music, it would sound the same across all instruments. Not always! Some instruments are what we call "transposing instruments". This means they read notes in one key, but sound in another.

For example, when a trumpet (which is commonly in the key of B♭, or B-flat) plays a written C, it actually produces a B♭ note. Similarly, a French horn (commonly in the key of F) playing a written C produces an F note. ​Check out this video to get a better sense of what this means sound-wise​.

This is confusing, to say the least! So, let's break down why different instruments are in different keys into a few understandable parts:

What is a key?

A key is a collection of notes that form the harmonic foundation of a piece of music. For instance, if a song is said to be in C Major, it primarily consists of notes from the C Major scale: C, D, E, F, G, A, and B.

What does it mean for an instrument to be in a certain key?

When we say that a trumpet is in the key of B♭, we mean that it most naturally produces sounds from a B♭ scale. For instance, if play a trumpet without pressing any valves, the sound produced would be a B♭.

So why are instruments in different keys?

There are a lot reasons why some instruments are in different keys, but here are three key reasons:

Reason #1: It's how instruments were historically built.

When instruments were originally invented, there were limitations in their design. For example, early wind instruments like horns were first built without any valves, so the sounds they produced were inherently limited to the natural key of the instrument. Over time, innovations have allowed for instruments to play in a wider range of keys, but now we are most familiar with hearing instruments in their original keys.

Reason #2: It makes it easier for a musician to play instruments of different sizes.

Different types of instruments, especially wind instruments like saxophones and brass instruments like trumpets and horns, come in various sizes. Larger instruments produce lower sounds, while smaller ones produce higher sounds.

If all saxophones played in the same key, musicians would have to learn an entirely new set of fingerings to play the same notes on different-sized instruments. By having different instruments in different keys, musicians can more easily switch between instruments of varying sizes without having to relearn fingerings.

Reason #3: It's easier to read sheet music.

Sheet music is designed to accommodate around 20 notes within and around the ledger lines. This keeps the music easy to read and play.

Instruments with natural keys that are far outside of this range would require their sheet music to have an excessive amount of ledger lines. This would make both reading and writing sheet music extremely difficult.

Transposing repositions the notes to fit comfortably within the ledger lines making life easier for composers and musicians alike.

In summary, the reason why instruments are in different keys boils down to the evolution of instrument design, the practicality for musicians switching between instruments, and the need to make sheet music more accessible and easier to read. These factors ensure that musicians can play their instruments effectively and comfortably, regardless of the key or the type of music they are performing.

Check out some other curious questions:

Sources

Estrin, R. (2018, September 3). Why are Instruments in Different Keys?. Living Pianos. https://livingpianos.com/why-are-instruments-in-different-keys/

How To Transpose Instruments FAST - TWO MINUTE MUSIC THEORY #23. (2017). Two Minute Music Theory. Retrieved September 24, 2023, from https://www.youtube.com/watch?v=17HXSSA0LHc

Konyi, M. (n.d.). What Does ‘In The Key Of’ Mean? A Helpful Beginner’s Guide. Piano from Scratch. https://pianofs.com/what-does-in-the-key-of-mean/

Mitchell, P. (2023, June 21). Why Are Band Instruments In Different Keys?. Sound Adventurer. https://soundadventurer.com/band-instruments-different-keys/