Does everything need to be in the same key for songs?

Does every instrument in a song have to be the same key?

If a song is in a certain key, can you add an instrument or vocal to the song / track as long as they are in the same scale of that key. So for example, if a song is in key D major, you can add any instrument/vocal as long as the key is in the D major scale, so E minor, B minor, G Major, A Major, etc, etc.

Can all songs be played in any key?

Theoretically you can play any melody and chord progression for a song in any key. But a song will often work best in certain keys and usually the key it was written in for a specific instrument.

Does it matter what key your song is in?

In other words, it's quite possible to generate interesting chord progressions even if you don't know the key of your song. Your ears will tell you if the progressions are working. So that's one answer: It doesn't matter if you don't know the key of your song; you can still come up with progressions that sound great.

Does a song have to be in one key?

There are exceptions to everything. A song would generally be regarded as being in the key of C major if it started out in C major, even if it ended in another key. Some songs stay in the same key all the way through, and some songs change keys.