Ray: The trajectory from a fixed point to a point moving in a certain direction is called ray. This fixed point is called the endpoint of the ray, and this endpoint is also called the origin. A ray has only one endpoint and can extend to one end indefinitely. Not measurable. (A ray can indicate its endpoint and any point on the ray with two capital letters: ray OA)
Line segment: The part between any two points on a straight line is called a line segment. These two points are called the endpoints of a line segment, which has a length and can be measured. (A line segment can be represented by capital letters of two endpoints: line segment AB, and it can also be represented by lowercase letters; Line segment a)
Properties of line segment: Among all the straight lines connecting two points, the line segment is the shortest.
Vertical parallelism: Two straight lines that do not intersect in the same plane are called parallel lines, or they are parallel to each other.
If two lines intersect at right angles, they are said to be perpendicular to each other, one of which is called the perpendicular of the other, and the intersection of these two lines is called the vertical foot.
Distance from point to straight line: a point outside the straight line is taken as a vertical line, and the length of the line segment from this point to the vertical foot is called the distance from point to straight line. The vertical line drawn from a point outside the straight line is the shortest.
Distance between parallel lines: draw a vertical line from a point on a straight line to its parallel lines, and the length of the line segment from this point to the vertical foot is called the distance between parallel lines. The distance between parallel lines is equal everywhere. In other words, the lengths of the vertical lines between parallel lines are all equal.