A | B |

Angle | A figure formed by two rays starting from the same endpoint. |

Vertex | The common endpoint of two rays. |

Protractor | An instrument used to measure an angle. |

Congruent angles | Angles that have the same measurement. |

Perpendicular lines | Two lines that intersect to form 90 degree angles. |

Right angle | A 90 degree angle. |

Acute angle | An angle that measures between 0 and 90 degrees. |

Obtuse angle | An angle that measures between 90 and 180 degrees. |

Complementary angles | Two angles whose sum is 90 degrees. |

Supplementary angles | Two angles whose sum is 180 degrees. |