This is a seam gauge. Do you have one?
If not, you really should buy one.
They are essentially mini rulers.
They are 6 or 6 1/2 inches long, depending on the manufacturer.
For small measurements, they are so easy to maneuver…much easier than a 12″ ruler, yardstick or measuring tape.
And, they have a little bonus feature:
There’s a plastic slide that you can move up and down the gauge.
This allows you to set the gauge at a desired measurement and keep it there while you cut the fabric all the way around a hem, or press a fold, for example.
I use mine every day for all sorts of alterations.
I also use it when I am pinning in the bust or side seams of a bridal gown or a blouse for a customer. I measure how much I took in and write it down in case the pin falls out.
Any time I need to take a small measurement, the seam gauge is right there on the battlefield with me.
It’s in my arsenal of top four necessary items I use every day.
Stay tuned for the other three.
The Sewing Garden 
Could you tell me where can I buy them? thank you
Yes. You can buy them at any fabric store or Walmart.
i have to do a school project. is reading a seam gauge the same as reading a ruler???
Yes, but a seam gauge, as you can see, is much smaller. Hope your project is fun and successful.