xkcd #3241: Horizontal Stabilizers
Title text:
It started as a mistake that everyone was afraid to admit to, and then it stuck because removing it ‘looks silly.’.
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3241/
Crucial, but depending on the design technically not required! There are designs of planes where the entire plane is just one big wing. Buuuuuuut having the traditional rudder, aileron, and elevator is tried trued and reliable.