|Pick not thy teeth at the table|
Spit not, cough not, nor blow thy nose at table; if it may be avoided.
That's one of the rules of good manners in the earliest known books on manners for children.
And that's pretty good advice for today, also.
The School of Good Manners 1787 was to help Parents teach children how to behave.
Conduct in 18th century English society was an indication of education and class. Books on conduct offered readers an opportunity to improve their lot in life by imitating the manners of the more well-to-do.
Some other rules are:
Come not to the table without having your hands and face washed, and your head combed.