What are hidden icons? I saw that term in Icons Handbook.
The original emoticons only existed as 19×19px and, while they had transparent backgrounds, the edges were aliased. An odd-numbered grid can allow you to centre elements better (see chapter 5), but the decision was taken to start at 20px to allow more straightforward scaling to 30, 40, 60 and 80px sizes, as well as design some new (hidden) icons. This meant some of the basic proportions of the eyes to head had to be changed.
'Hidden icons' have a long history. What does it mean?