Open hole

An empty hole in a wind-instrument fingering diagram means that the indicated tone hole or key is open. The diagram's layout, not the circle alone, identifies the physical hole.

Open hole: music notation symbol placed above the note. Leave the hole open.
Family
Wind and brass signs
Where it sits
Above the note
What it looks like
A circle

Do not confuse it with

A filled circle means closed; an ordinary small circle above a staff note may instead be a harmonic or other technique.

What the sources say

All notation signs

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