The black bar with a number above it on each photo is there to show the scale (size) of each microorganism. The number is how long the black bar is in micrometres. One micrometre is commonly known as one micron. Micron is also written as µm or just µ. How big is a micrometre? A micrometre is one millionth of a metre. So a metre is chopped up into one million pieces. How I think of it is like this… If you take a ruler and look at one millimetre (the smallest marking on the ruler) and then imagine it divided into 1000 pieces: each of those pieces is 1 micron or 1µm or 1µ long. So half a millimetre will be 500µ long.