They're the things which separate the civilised from the animals. Those elements of behaviour that are so fundamental to our functioning as a coherent society that no one has ever felt the need to codify them.
And yet, still some people break them.
Well, it's time to fight back, and this excellent thread on Reddit has sought to finally nail down the most important of those crucial unwritten rules that make the world a better place.
Below are the top 15 rules: if everyone could use these as your gospel (alongside at least a few of the Ten Commandments), then we'd all be very happy.
Any they've missed? Let us know in the comments.