Write the Word or Phrase That Best Completes Each Statement $$\text { If } m _ { 1 } \text { and } m _ { 2 } \text { represent the slopes of two nonvertical perpendicular lines, then } m _ { 1 } m _ { 2 } =$$

Write the word or phrase that best completes each statement or answers the question.
Provide the missing information.
- $$\text { If } m _ { 1 } \text { and } m _ { 2 } \text { represent the slopes of two nonvertical perpendicular lines, then } m _ { 1 } m _ { 2 } =$$ .