"a major key" is a correct and commonly used phrase in written English.
You can use it to refer to an important point, idea, or element. For example, "The major key to success is having a positive attitude."
That's love in a major key.
Simple-minded, blocky, smug, uplifting, always in a major key.
The language is therefore staccato, empowered, in a major key . . .
Hair styles are a major key to the desired effect.
The piece, a simple, poignant string essay, also ends optimistically in a major key.
He played a simple study in a major key then played it in a minor key.
It allowed us to move from a minor key to a major key.
At times adopts a major key in instruments, then goes back to the minor key.
In fact, health care is a major key to the possibility of an improved economy.
What a player does when the serve is returned is a major key to winning matches.
Monitoring the deformation of both natural phenomena and man-made structures is a major key to assessing structural dynamic responses.
