De civilitate morum puerilium libellus

Yawning.

Laughter.

If a yawn happens to press upon you, and you are not able to turn away or withdraw, cover your mouth with a handkerchief. To laugh at everything said or done is for fools: to laugh at nothing is for the dull-witted. To laugh at obscene words or deeds is wickedness. The cachinnus a loud, cackling laugh and that excessive laughter which shakes the whole body, which for this reason the Greeks call synchrosio a general noisy laughter, is becoming to no age, let alone childhood.

Neighing.

It is unsightly that some people emit neighing sounds while laughing. That person is also indecorous who pulls the mouth wide, with cheeks wrinkled and teeth bared, which is dog-like and is called a Sardonic laugh.

Cheerfulness of the face.

The face should express cheerfulness in such a way that it neither dishonors the habit of the mouth nor proves a dissolute mind.

The voices of fools.

Those are the voices of fools: "I am flowing with laughter," "I am bursting with laughter," "I am dying of laughter." If such a ridiculous thing occurs that it expresses this kind of laughter in you against your will, the face should be covered with a handkerchief or hand. To laugh alone, or for no evident reason, is attributed either to stupidity or insanity. If such a thing should arise, it is an act of civility to reveal the cause of the laughter to others, or if you do not think it should be brought forward, to invent something so that no one suspects they are being mocked.