In mathematics and physics, a vertical line is a direction that passes through the local gravity centre of a point (i.e. a point that is in the ‘down’ quadrant) and is parallel to the y-axis. For example, a vertical line passing through the point (6, 0) has the equation x = 6.

In drawing, it is generally tirage vertical that the correct spatial positioning of features on an image is critical to producing accurate depictions of objects. This is because an object’s correct relative position depends on the skillful co-ordination of perceptual, motor and decision-making processes. It is therefore not surprising that most people are poor at generating realistic drawings.

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For example, when producing a portrait, it is commonly reported that people tend to place the eyes too high on the head. This systematic bias can be attributed to the effects of feature salience accounts, which suggest that people pay more attention to salient features and regions in a scene, and allocate them a greater proportion of space within the outline of an object compared to less-important features and regions.

Similarly, when drawing a building, it is common to produce a two-point perspective by converging the parallel lines forming the fronts of the building at a point on the horizon that may not be on the drawing paper. This is a reflection of the fact that most buildings have their fronts oriented to face towards the viewer. A similar bias exists when drawing a landscape whereby people typically draw peaks too close to each other.