In the old daes, a "disk pak" had a bunch av disks on a rod. All tern tugether.
& yu hav a bunch av muevabel arms that kan go in/out tween
senter & rim av a disk. Tu make thangs simpler, em muev all arms tugether
so all R same distans from senter. Eech hed reed same trak on its disk...
a serkel on its disk, distans=x from senter.
The data from all such heds at 1 distans was kalled a "cylinder"