The concepts of horizontal and vertical are widely used in life. For example, construction workers need to use a level to ensure the verticality and levelness of the wall when building a wall; Geographers need to calculate the height and vertical depth of a point on the earth relative to the horizontal plane; When the missile is launched, it needs to be stable in the vertical direction to ensure the correctness of the flight trajectory; Automobile manufacturers need to ensure that the wheels are vertical to the car body and the car body is horizontal to the wheels to improve the handling and safety of the vehicle.
It is also widely used in mathematics. For example, in rectangular coordinate system, we usually express the X axis as the horizontal direction and the Y axis as the vertical direction, so that we can describe the position and direction of points, line segments and vectors in the plane. In addition, we can also use this concept to solve various geometric problems, such as the intersection of two straight lines on a plane, the properties of parallel lines and perpendicular lines, the measurement of angles and so on. The relationship between vertical and horizontal trigonometric functions is also very important. Such as sine, cosine and tangent functions, are closely related to vertical and horizontal straight lines.