The Do which is placed in the center of the clef is called "Middle C".
We sometimes see C4 and C5 as a pitch name of middle C.
What are they?
C4, C5 style pitch name is for COMPUTER MUSIC.
This style is used in music production software like MIDI sequencer.
The Default setting depends on the software.
But, most software have the preference setting of changing the middle C.
It seems that setting middle C as C4 is more popular than setting middle C as C5.
So, you don't have to care much about C4,C5 style when you play the musical instruments and study music.
