In 1923 Kandinsky proposed a universal correspondence between the three elementary shapes and the three primary colors: the dynamic triangle is inherently yellow, the static square is intrinsically red, and the serene circle is naturally blue.
The series ▲■● represents Kandinsky’s attempt to prove a universal correlation between color and geometry; it has become one of the most famous icons of the Bauhaus. Kandinsky conceived of these colors and shapes as a series of oppositions: yellow and blue represent the extremes of hot/cold, light/dark, and active/passive, while red is the intermediary between them. The triangle, square, and circle are graphic equivalents of the same polarities.
In March 2021, I went through a fun self-imposed experiment: no emoji for 2 weeks. Not on social media, not in private messages, not even as Slack or Discord reactions. No emoticon either: the goal was to communicate without illustrations, only with words. I did a semi-rigorous (a.k.a. half-assed) diary study, taking notes on my feelings and behaviour.
Perceptual: "They couldn't figure out what to do next", "they couldn't find the feature", "they didn't know they could click that button..." etc.
Domain-specific: "We need a way to jump back here because in their workflow this happens..."
In general, usability testing only catches type 1 perceptual problems. Because in those tests you take people out of the real world and assign them tasks. Usability testing doesn't catch domain-specific problems because they only come up in real life use.