The treble clef is known even to people far from musical art. This sign is placed at the beginning of the musical staff, as if opening it, because it is called the key.
In modern musical recordings - notations - a musical staff consisting of five lines is used. The notes are located both on the rulers and between them.
In this way, only eleven notes can be placed on a stave, no more. It's less than two octaves, and musicians use a lot more. How to record all the other notes? True, additional rulers are also used above and below, but if there are more than four of them, it becomes very difficult for a musician to navigate. Here, special signs come to the rescue - keys.
Key value in musical notation
A musician, looking at a musical staff, knows exactly where a note is located. This becomes possible because he has a starting point: between the second and third rulers is for the first octave. Therefore, a step higher - on the third ruler - there will be a note si of the same octave, and on the second - salt, etc.
But any reference frame is very arbitrary. If you change the starting point, the whole system will change. So, without resorting to a large number of additional rulers, you can be in any octave.
That is why the Italian music theorist Guido d'Arezzo, who laid the foundations of modern notation, came up with special signs - keys. Their purpose is to indicate a reference point on the stave, that note with respect to which all the others are determined.
