Sources

Where the facts come from

Every factual claim on this site links to its source. This is the full list.

How we checked

Each fact was written in our own words alongside its source. Our build script stores the text of every source and, for each fact, checks that the key words and figures really appear in it. 137 of 137 checks passed when this site was last built (13 November 2026). Where we couldn’t find a named source saying something, we left it out. Where good sources disagree, we say so and give both.

Where sources differ

The title of the 1958 paper. “Impossible objects: a special type of visual illusion” (Escher museum, MathWorld) or “…a special type of illusion” (Michael Bach, the Illusions Index and the 2020 Journal of Imaging study, in their reference lists). We use the Escher museum’s wording.
Who drew the impossible triangle first. Reutersvärd in 1934 (Escher museum, Illusions Index, MathWorld); the Penroses found it independently and published it in 1958. Michael Bach adds that Josef Albers (1931) and Hogarth (1754) made impossible pictures earlier still, and knows one from around 1025.
Where Reutersvärd drew it. While sketching cubes in perspective (Escher museum), during a “long lecture” (MathWorld), or, by an anecdote the Illusions Index calls widely reported, in a Latin class.
The cube in Belvedere. The Escher museum separates the Necker cube drawn on the floor from the impossible cube in the man’s hands; a 2020 study names the Necker cube as the impossible structure. Michael Bach says impossible figures are not the same thing as the Necker cube’s flipping.
Whose idea the Perth sculpture is. A concept by Lionel and Roger Penrose “in 1954” (WA Government, 1999), devised by Sir Roger Penrose in the 1950s (local art trail), or the triangle “first articulated” by Reutersvärd (the makers, DENMAC).
Where it stands. The roundabout at Royal Street, Brook Street and East Parade (WA Public Art Inventory) or Claisebrook Square (Illusions Index).
When. Completed in November 1999 (DENMAC); unveiled on 5 November 1999 (WA Government).
How many viewpoints. Two spots for the Perth sculpture (local art trail); one position for Richard Gregory’s 1968 wooden triangle (Illusions Index).

How we checked

Where sources differ

All sources (25)