Knitters use various tools to accurately measure their work, their stitches, and their needles.
Knitting needle gauges are flat knitting notions with holes in varying sizes. The holes correspond with the size of different needles, use the holes to measure the size of a knitting needle. Most gauge tools will have both the US sizes and the mm metric sizes on them.
Stitch gauges are flat knitting notions with an opening of a certain size that you can lay on your work to count how many stitches and rows you make in that increment.
These tools are invaluable when you are making a gauge swatch before you start your project.
Yarn gauges will help you determine how many WPI (wraps per inch) your yarn is. Spinners and weavers love these tools.
Most of these gauges will also display a ruler that shows inches and centimers.
You will also find tape measures here.