Which of the lines on the right, is the continuation of the black line on the left?

red one
blue one
none of them
both of them
This is known as Poggendorff illusion, first described in 1860. Many detailed studies of the illusion point to its principal cause: acute angles in the figure are seen by viewers as expanded.
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Answers by country

red one
blue one
none of them
both of them
answers
flagSwitzerland
56%
16%
12%
16%
25
flagthe Netherlands
53%
6%
6%
33%
15
flagthe United States
47%
18%
20%
12%
367
flagIndia
45%
22%
17%
14%
163
flagthe United Kingdom
43%
20%
19%
16%
139
flagthe Philippines
43%
18%
25%
12%
16
flagGermany
42%
24%
20%
11%
308
flagCanada
42%
23%
28%
4%
21
flagRepublic of Ireland
42%
10%
31%
15%
19
flagPoland
41%
20%
22%
15%
14127
flagUkraine
41%
16%
26%
15%
154
flagAustria
41%
18%
24%
15%
58
flagUnknown
40%
18%
27%
13%
22
flagBelarus
39%
14%
39%
3%
28
flagFrance
38%
23%
15%
23%
13
flagRussia
36%
18%
29%
13%
144
flagGreece
33%
19%
14%
33%
21
flagKazakhstan
28%
28%
28%
14%
21
flagAustralia
25%
18%
25%
25%
27
flagUzbekistan
21%
7%
28%
42%
14