Which of the following must you do before you change lanes?

You must always use your turn signals, check your mirrors, and look over your shoulder before you change lanes. Changing lanes can be hazardous. Your car has large blind spots to the left and to the right. These blind spots are large enough to hide another car. That is why you must always signal, check your mirrors, and look over your shoulder. If you don't, you may discover the car next to you the hard way. [When Changing Lanes, Seeing Well, Wisconsin DOT Motorists' Handbook], [Signal When You Change Direction, Letting Others Know What You Are Doing, Communicating, Wisconsin DOT Motorists' Handbook]