Mouth shape

For singing classical music, a protruded shape of the mouth is recommended.
So your lips are rounded a bit, like when pronouncing the “O”-sound.

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For singing pop, gospel, rock, etc. the mouth shape is a bit wider.

Place the corners of your lips forward a bit, at the level of your eyeteeth.

This protruded mouthshape helps your throat to open up and gives you a warmer sound.

When you protrude your mouth, your throat will widen. You can feel that when you start on a wide “EE” and change your mouth shape gradually into a “OO” position. In the “OO” position, you will feel some muscles becoming active in the back of the throat (at the level of your ears). This light activity of muscles keeps your throat opened.
Watch the video. Cecilia Bartoli keeps het mouth relavitvely small while singing. The only exeption is when she sings high notes.

Not all singers use the protruded mouth shape as intense as others.
For some singers a protruded mouth leads to too much tension of the lower jaw.
So when you protrude your lips, make sure your lips do the work and the muscles of your jaw do not!