Elbows on tables

If you look at the one or two histories of etiquette, this had a very practical implication. Originally, tables were trestles. If you rested your elbow on the table, you were in danger of possibly overturning the trestle with all the resultant hassle and general mess.

Many of the other things which are now regarded as "manners" were originally practical in this way.

Cheers!
Ian