In order to create a horizontal 2-inch-thick dotted green line exactly 2 inches away from the top of the canvas, a graphic artist would most likely need to use

A ruler, surely, or one of those measurey things. Sorry if not helpful.

the answer to your question would be something called hertz. the unit of measurement called a hertz, which is one cycle per second, is used to measure clock speed. within one cycle, your computer processes, refreshes, or updates itself to the changes that it is going through. in the case of computer clock speed, one hertz equals one tick per second. the clock speed of computers is usually measured in megahertz (mhz) or gigahertz (ghz)