Triangles, Why Triangles?

I don't think that there was any reason for triangles. I think it was a joke that his paintings were pretty simplistic and nothing special, which is further pointed out when Malkovich says something to the effect "It took me 25 years to be able paint something that great"

ahahaha
Wow, I haven't seen this movie for awhile but just the subject of this post sent me bursting out in laughter. Malkovitch was so great in this film.
Somewhat ironically, triangles are getting very trendy right now especially in the whole electro/house scene. Not fine art of course but in the graphic design world at least.

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I think what you have to remember is that at the time the minimalist were being major rebels with what they were doing just as any other movement throughout art history had the same basis. By todays standards we look at those and say who cares. it's been done it's not really shocking any more, but the same could be said about pretty much any artist since art began.

Excellent points. I suspect the intention was to show how meaningless his life had become, that he spent 25 years to achieve a style that you'd see on beach towels at Target.