Horizon line can be regarded as your eye-level line, the level which you are intended to view the painting at, and for an accurate rendition of recession within the painting. The horizon line is an important part of a painting's compositional arrangement, it is important in arranging a realistic scene and the line needs to be straight as the artist then applies perspective rules to objects within the painting in relation to this line.
The horizon line is an imaginary line to which all converging lines recede (in other words where all things recede). However in a fictive space, that is a painting's perspective, it is the level of the viewer's eye in relation to the scene of the painting. In the real world, the horizon line is where the sky meets the sea. The horizon line in art is a perspectival line. A simplistic view of the horizon line in art, is to consider it as your 'eye-level line'. Oblique (or angular) perspective: two vanishing points, located on the horizon line.
