The vertical line appears longer than the horizontal line, even though both are of the same length.
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Illusion Credit

Johann Joseph Oppel (1815 – 1894), German mathematician and physicist


Look at the image on the left, and note how the vertical line appears longer than the horizontal line. Then hover your cursor over the image and see how the rulers demonstrate that the two lines are the same length.


The vertical line appears longer than the horizontal line when it is not. 

The vertical line appears longer than the horizontal line, even though both are of the same length.
Media Licence:

Illusion Credit

Johann Joseph Oppel (1815 – 1894), German mathematician and physicist
  • Vertical-Horizontal Illusion

The Vertical-Horizontal Illusion (also known as the Bisection illusion) was created by Johann Joseph Oppel (1815 – 1894), a German mathematician and physicist. The Vertical-Horizontal Illusion was first published in the journal Jahresbericht des physikalischen Vereins zu Frankfurt am Main in 1855.

The Vertical-Horizontal Illusion comes in different versions, including an “L” and “+” configuration, but the inverted “T” is the most common.

The Vertical-Horizontal Illusion is one among a number of illusions where a central aspect of a simple line image—e.g. the length, straightness, or parallelism of lines—appears distorted by other aspects of the image—e.g. other background/foreground lines, or other intersecting shapes. These are sometimes called ‘geometrical-optical illusions’. You can search for other geometric illusions in the Illusions Index.

One explanation for how the Vertical-Horizontal Illusion works is the ‘misapplied size constancy scaling’ hypothesis which is also used to try and explain the Müller-Lyer Illusion. According to this explanation, the intersecting lines engage the part of the visual system that deals with depth cues in retinal images, and results in the vertical line being perceived as longer because it is processed as being further away (Gregory 1997). Others have proposed explanations which appeal to different parts of the visual system, including the eye (for discussion see Wolfe et al. 2005).

Some have argued that those who have been raised in ‘carpentered environments’ which contain lots of right-angles (e.g. many urban environments) are more susceptible to the illusion (e.g. Mast et al. 2004). Others have suggested that other factors, such as gender, may play a role (e.g. Ramsjou et al. 1999). The jury remains out on such hypotheses.  

The Vertical-Horizontal Illusion is interesting because it is relevant to debates about modularity, cognitive penetration, and the nature of experience. To explain: on the hypothesis that the mind is modular, a mental module is a kind of semi-independent department of the mind which deals with particular types of inputs, and gives particular types of outputs, and whose inner workings are not accessible to the conscious awareness of the person – all one can get access to are the relevant outputs. So, in the case of the Vertical-Horizontal Illusion, a standard way of explaining why experience of the illusion persists even though one knows that one is experiencing an illusion is that the module, or modules, which constitute the visual system are ‘cognitively impenetrable’ to some degree—i.e. their inner workings and outputs cannot be influenced by conscious awareness. For a general discussion of cognitive penetration, see Macpherson (2012).

Philosophers have also been interested in what illusions like the Vertical-Horizontal Illusion can tell us about the nature of experience. For example, in the case of experiencing the Vertical-Horizontal Illusion, it would seem to be that one can know that the two lines are the same length whilst at the same time experience them as different lengths. If so, then this might count against the claim the perceptual states are belief-like, because if perceptual states were belief-like then, when experiencing the Vertical-Horizontal Illusion one would simultaneously believe that the lines were, and were not, the same length. This would seem to entail that one was being irrational, because one would simultaneously be holding contradictory beliefs. But it seems highly implausible that one is being irrational when under going this illusion. For discussion of this general point about whether perceptions are like beliefs, see Crane & French (2016).


Crane, T., and French, C., 2016. The Problem of Perception. In: Zalta, E. N., ed. The Stanford Encyclopedia of Philosophy. Metaphysics Research Lab, CSLI, Stanford University.

Gregory, R.L., 1997. Eye and brain: The psychology of seeing, 5th edition. Princeton University Press.

Macpherson, F., 2012. Cognitive penetration of colour experience: Rethinking the issue in light of an indirect mechanism. Philosophy and Phenomenological Research, 84(1), pp.24-62.

Mast, F.W. and Oman, C.M., 2004. Top-down processing and visual reorientation illusions in a virtual reality environment. Swiss Journal of Psychology, 63(3), pp.143-149.

Oppel, J. J. 1855. Über geometrisch-optische Täuschungen. Jahresbericht des physikalischen Vereins zu Frankfurt am Main, 1854–1855, pp.37–47.

Rasmjou, S., Hausmann, M. and Güntürkün, O., 1999. Hemispheric dominance and gender in the perception of an illusion. Neuropsychologia, 37(9), pp.1041-1047.

Wolfe, U., Maloney, L.T. and Tam, M., 2005. Distortions of perceived length in the frontoparallel plane: Tests of perspective theories. Perception & Psychophysics, 67(6), pp.967-979.

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Please cite this article as follows:

Donaldson, J. and Macpherson, F. (July 2017), "Vertical-Horizontal Illusion" in F. Macpherson (ed.), The Illusions Index. Retrieved from

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This article is licensed under Creative Commons (CC BY-NC_SA 4.0)

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