A variation of the Poggendorff illusion in which there were no closed angles was found to correlate highly with the classical Poggendorff figure. In addition, repeated trials had a similar effect on both illusions. It was concluded that explanations of the Poggendorff illusion which focus on the presence of closed acute angles are probably incorrect.

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Burns and Pritchard's (1971) explanation of the Poggendorff illusion is criticized. An experiment was designed to determine whether the acute angle plays any role in the perception of the illusion. The results showed that (i) an inducing line which crossed a test-line was highly effective in altering the apparent orientation of the test line, (ii) an inducing line forming an acute angle with a test-line had a small effect in changing the apparent orientation of the test-line, and (iii) an acute angle which formed part of the Poggendorff configuration produced an effect opposite to that predicted by the view that acute angles are perceptually enlarged.

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