The embouchure and the lining of head joints
The embouchure - the ″mouthpiece″ - of the flute is extremely important in determining the quality of the sound and the comfort of playing. Small differences in the embouchure completely change the feeling and sound of an instrument. There is, however, no such thing as a general ideal embouchure and finding an embouchure that suits you is very much a question of personal taste and preference. This does not mean that there are no good and bad embouchures, but preferences in size and shape are not the same for all players.
Size of the embouchure
One of the variables of the embouchure is the size of the hole. A smaller embouchure will produce less volume, but it also needs less air. The sound will generally be a bit more closed and reedy. The bottom notes as well as the second octave tend to come out more easily.
A larger embouchure will need more air, or require a better control of the air stream by the lip muscles. The sound will be more open and brilliant, but the effect of a smaller embouchure can be approached by covering the embouchure more with the lips and maintaining a tight control over the air stream.
Undercutting of the embouchure
A feature of traditional embouchures is that they are undercut. This means that they are made in such a way that they become larger towards the bore of the instrument. Baroque embouchures are very much undercut, whilst modern flute embouchures are often not undercut at all. The amount of undercut will influence the playing in a complex way. Simplifying things, one can say that - within certain limits - more undercut will give more dynamic to the flute. It will make the second octave sound more easily and in general give a more open sound.
The choice of the embouchure
For inexperienced players my advice would be to go for a standard medium sized embouchure, or to ask for a smaller embouchure if volume is not that important to you and you want to spare some air.
Unless specified otherwise, my flutes come with a standard undercut medium sized oval embouchure. More experienced players can come to the workshop to try different types of embouchures.
Lining of the head joint
A head joint that is lined with metal will produce a tone with more harmonics. It will also be a bit louder than a head joint that has no lining. The unlined head joint will produce a sound that is more ″woody″. Neither is better than the other, the choice is a matter of personal preference.