Before rulers, people measured with their bodies. A foot was a foot. A cubit was elbow-to-fingertip. Try measuring your room two ways: hands first, then a ruler.
Why standardised units matter
Your hand and mine are different sizes. Trade and engineering broke until we agreed on a metre. Today, the metre is defined by the speed of light — same for everyone, everywhere.
1 metre = the distance light travels in 1/299,792,458 of a second. Defined since 1983.