Under the microscope, skeletal muscle is crossed by stripes. These are called striations, and muscle that shows them is striated muscle.
The stripes appear because every myofibril packed inside the fibre has its dark region in the same place, and those places line up across the bundle.
Four myofibrils are drawn side by side. If they did not line up, the dark regions would cancel each other out and the tissue would look a uniform grey.
This figure is closer to the microscope image. The edges are blurred because that is how they actually look.
Dark and light bands alternate, and the middle of each dark band is lighter again. The narrowest vertical lines are the divisions between neighbours.
One period of this stripe is the sarcomere. Redrawing the same thing as a diagram makes it clear what is dark and what is light.
This is the diagram. The thick vertical lines at each end are Z lines, and one sarcomere runs from a Z line to the next.
The thick bar lying along the middle is the thick filament, and the wavy strands reaching in from both Z lines are the thin filaments.
The small projections coming off the thick filament are the myosin heads, angled towards the thin filaments. What those heads do to their partner comes in a later article.
The stretch occupied by the thick filament is the A band, also called the dark band. It looks dark because the thick filament is there.
From the Z line to the edge of the A band is the I band, also called the light band. Only thin filament is present, so it looks pale.
In the middle of the A band, the part the thin filaments have not reached is the H zone. That is why the centre of the dark band is lighter again.
These three names are all there is to learn. The next article watches what happens to them when the length changes.