So the z line number one and the z line number two are basically the boundary lines for our sarcomir. So the sarcomir begins on this z line and ends on the z line, while the second sarcommer here begins on the z line and ends on the other z line that is not shown. And it's the z line that basically gives skeletal muscle as well as our cardiac muscle striped or its striped look. So if we look on the microscope, our muscle will basically look like it's striped. And that's because of these z lines. So when a muscle actually contracts, the way that our sarcomir contracts is in the following way, as we'll see in the next lecture, these mycin heads basically attach to our actin, to our thin filament.