Determination of parallel lines: the topic is that the isosceles angle is equal (or the internal angle of the same side is equal or complementary), and the conclusion is that two straight lines are parallel. In other words, parallelism is derived from conditions.

Properties of parallel lines: Assuming that two lines are parallel, the conclusion is that the congruent angle is equal (or the internal angle and the internal angle on the same side are equal or complementary). That is, draw conclusions from parallelism.