There are certain facts you learn when you start out in the plumbing profession: hot on the left, it runs downhill; payday is on Friday; and wear an orange hat around an open septic tank. (That last one is my father's. He always said if you fall in, we'll know where to find you.)
Another important fact: You need to pitch the drain enough so the solids don't fall out of suspension. And don't pitch it too much, or the liquids run away from the solids. One of the big debates always was 3-inch pipe. Can you pitch it 1/8 inch per foot, or does it have to be 1/4 inch per foot? Plumbing codes still disagree on that one. But there is a correct answer. I'll give it to you at the end of the column.