The original low density disks have now disappeared, leaving only double and high density disks. This results in some confusion for new users as the name double density seems to imply it holds more than the other current type - high density. Double density diskettes are no longer common. In the past couple of years the density was increased and disks that can hold 2.88M appeared on the market but they are not widely used. The total capacity of a disk is a multiplication of surfaces by tracks by sectors.
Densities are an issue only when using older computers that have double density disk drives.
High density disk drives will read and use any density disk. However, computers sold before about 1990 often had double density disk drives and these cannot read high density diskettes. Top of Page Diskettes rotate within their permanent jacket at either 300 or 600rpm. The jacket is lined with a fabric to clean the disk as it spins and to prevent the friction of plastic rubbing against plastic. All disk drives have indicator lights that show when the disk is actually being used.