I found out the other day why washrooms are often labelled “WC” on floor plans. I thought to myself “Shouldn’t it be ‘WR’ if it’s a washroom?” but IT TURNS OUT that “WC” stands for “water closet”! Whoa! As to why they call them water closets… no idea. Maybe I’ll look that up one day.
Also, the bottom (lowest point) of a pipe is called the pipe’s “invert”.