Etiquette says that if somebody at a table asks for the salt shaker, you should also pass the pepper, and vice versa? This changes if there is a pepper mill or a salt cellar (a small bowl); because they are more cumbersome, they can be passed alone.
It is more common these days to see shakers, so here's what you need to know about them. The one with more (and typically smaller) holes is the salt (because it is finer), and the one with fewer, larger holes is the pepper.
If you see a set with 1 and 2 holes, the one with 2 is the salt. If a set has 3 and 5 holes, the one with 5 is the salt. Got it?
Since more people use salt than pepper and most people are right-handed (sorry, lefties), the salt should be placed to the right of the pepper on the table. But remember, when one is requested at the table, as a courtesy to the diner, you should pass them together.
If you should be at a meal where they have a salt cellar instead of a shaker, there should be a spoon to use to get a bit of salt and sprinkle it onto your food. If there is not a spoon, etiquette says that you can use your thumb and forefinger to grab a pinch from the bowl.
Pepper is less likely to make you feel the urge to sneeze if it's in a shaker than a mill or a bowl.
It's true! So let's see if they created these S&P sets correctly (given that a lot of people don't really know this etiquette well):
Not Correct, but it's okay! I have these cute little ghost salt & pepper shakers. They're not correct hole-wise, but they are so cute, we can forgive them. Plus, it's obvious that the salt should go in the white one...unless you're looking for a little trick to add to your treats.
The shaker on the left has pepper, everyone knows. I just hope with only one hole, that the pepper flows. Correct (although it looks like salt is coming out the bottom, of both- how odd.)
Now for a little joke. Two peanuts were walking down the sidewalk and one was a-salted. Okay, take that one with a grain of salt. Hey! Don't make fun of me- I'm insalted! Alright, enough of that. I'm glad you're here and humbled by your visit. You know, you're just the salt of the earth. Okay, no more. I mean it. Later, gators!