the products. Airplane engines cannot be put into reverse until the landing gear
has touched down, and cars cannot be put into reverse while traveling forward
at faster than five miles per hour. Similar principles can be applied to interactive
systems-for example, graying out inappropriate menu items so they can't be
inadvertently selected, or allowing web users to simply click on the date on a
calendar instead of having to type a month and date for a desired airline flight
departure. Likewise, instead of having to enter a 10-digit phone number, cell
phone users can scroll through a list of frequently or recently dialed phone numbers
and select one with a single button. Another option used by some systems,
such as the Visual Basic programming environment, is to offer automatic command
completion to reduce the likelihood of user errors: the user types the first
few letters of a command and the computer completes it as soon as the input is
sufficient to distinguish the command from others. Techniques such as these do
some of the work for the user, thereby reducing opportunities for user errors.