Horizontal line

The horizontal or level line is parallel to the horizontal projection plane and oblique to the frontal plane projection.

It has only a frontal stroke. This line can open to the left or to the right, which is considered on the side where the distance is positive.

Frontal line:
The frontal projections of a horizontal line are parallel to the x-axis, the horizontal ones are oblique.

The frontal line in projections:

The Line f has positive spacing and opens to the right and corresponds to the one represented in perspective. Line b has a negative spacing and an opening to the left is only represented by its
projections. 

The horizontal projections of a frontal straight line are parallel to the x-axis, the frontal ones are oblique.