U.S. System
These are the most common U.S. Standard Unit measurements:
 Inches
 Feet
 Yards
 Miles
The last joint of your finger or thumb is about 1 inch (depending on how big your fingers are!).

A long time ago, people had to use their own feet to measure things. However, since everyone has different sized feet, it did not let everyone understand the size or distance in the same way.
Using 12 inches put together to make feet allows for everyone to have a more accurate picture of what exactly a "foot" of length is. 
When 3 feet are together, this is called a yard.
The length of this guitar is about 1 yard. 
Miles are long distances and are mostly used to measure the distance between places which are far away from each other. Most people refer to miles when they are driving, biking or jogging.

Final thoughts about measuring length:
1 foot = 12 inches
1 yard = 3 feet
1 mile = 1,760 yards
1 yard = 3 feet
1 mile = 1,760 yards