In the same plane, there are two positional relationships between two straight lines: intersecting and parallel. If two straight lines have only one common point, they are said to intersect.

(2) Vertical line

One of the four angles formed by the intersection of two straight lines is a right angle, that is, the two straight lines are perpendicular to each other, one of which is called the perpendicular of the other straight line, and the intersection point is called the vertical foot.

(3) Equilibrium angle

Two straight lines A and B are cut by a third straight line C (or the intersection of C of A and B). On the same side of the cutting line C, cut the corners on the same side of the two straight lines A and B.. We call these two angles congruent angles.

(4) Internal dislocation angle

Two straight lines are cut by a third straight line, and the two corners are on both sides of the cutting line and sandwiched between the two cut straight lines. Diagonal lines with this positional relationship are called inscribed angles.

(5) ipsilateral internal angle

The two angles at which two straight lines intersect with the third line are called inner angles on the same side, which are located on the same side of the cutting line and within the cutting line.

(6) Parallel lines

In geometry, two straight lines that never intersect (and never coincide) on the same plane are called parallel lines.

The nature of parallel lines: ① Two lines are parallel, and the included angle is equal; ② Two straight lines are parallel and the internal dislocation angles are equal; ③ The two straight lines are parallel and complementary.

(7) Translation

Translation means that all points on the map move equidistantly along a straight line in the same plane. This kind of graphic movement is called graphic translation movement, which is called translation for short.