The Apple Disk II stored 140 KB on single-sided, "single-density" floppy disks, but it was very common for Apple II users to extend the capacity of a single-sided floppy disk to 280 KB by cutting out a second write-protect notch on the side of the disk using a "disk notcher" or hole puncher and inserting the disk flipped over. Double-sided disks, with notches on both sides, were available at a higher price, but in practice the magnetic coating on the reverse of nominally single-sided disks was usually of good enough quality to be used (both sides were coated in the same way to prevent warping, although only one side was certified for use). Early on, diskette manufacturers routinely warned that this technique would damage the read/write head of the drives or wear out the disk faster, and these warnings were frequently repeated in magazines of the day. In practice, however, this method was an inexpensive way to store twice as much data for no extra cost, and was widely used for commercially released floppies as well.