"Larger holes = Salt
Smaller holes = Pepper (the pre-ground type that @NoThankyou
I am sorry. That is backward. In general pepper needs larger holes because the pieces are larger. That is why it is so frustrating in restaurants that use one size shaker for both. I have to open the pepper shaker to get the pepper out, which I then pour into my left hand and then use my right thumb and fore finger to take up and sprinkle over my food. Now if you are using sea salt, then the those grains are generally larger.
Go into a kitchen store and look at the better sets of salt and pepper shakers. You will be able t4o see the difference.
I learned the salt/pepper shaker convention (salt - larger and more holes, pepper - smaller and fewer holes) from my mother in the 1960s, and there is little doubt that she learned it from her parents. I distinctly remember my mother teaching me the lesson, after I had put pepper on my food as a child, when I wanted salt. It was a large family gathering on Thanksgiving. If I worked on it, I could probably figure out the year.
My spouse learned the same thing from her family, probably around the same period. @NoThankyou
learned the same lesson, I know not when.
That said, yesterday I got out a 10x jeweler's loupe and examined the salt and pepper for myself. @GeorgeC
is right about one thing - the salt crystals are smaller than the pepper grains overall. The salt is much more consistent, mostly in small, uniform cubes. The pepper granules are irregular and range in size, but most of them are larger than the salt.
This morning I remembered a gift I received from one of my children last year - a digital microscope. What a fun, useful gadget! I got it out this morning and took a closeup of the salt and pepper. See the attached photo.
My spouse just walked in and noticed my setup. She believes that the reason we were taught to put salt in the shaker with larger and more holes is that people want more salt on their food than pepper. The shakers with smaller/fewer holes help people control and limit the amount of pepper they put on their food. The practice dates back to the days before people tried to limit their salt intake.
The next time I am near a kitchen store, I will look at the salt and pepper shakers, as recommended by GeorgeC. We have Williams-Sonoma and similar stores in our area. You got me; I am very curious to see for myself.
Our salt and pepper shakers are kept next to the stove, for use while cooking. We keep a salt shaker and pepper grinder next to the table. When people want pepper, they grind it on their food, like fancy restaurants. I bet that those granules are much larger than the fine commercially ground pepper.
Photo: Closeup of salt and commercially ground pepper granules on a painted green metal pullout from a tanker desk. The background looks white because of the shiny reflection of the LED light from the microscope.