Horizontal: different groups

Vertical: group size (darker: smaller)

Click a country on map to see its flag and flags of similar countries.

An exploration of flag similarities

How to read the colors: countries with similar flags are painted the same color. The bigger the group is, the brighter the group color. Countries with unique flags are black.

There's a great discussion on Quora about flag similarity. Here, an algorithm was used for grouping images together. There are several metrics of comparison: color (all red flags), shape (how are they partitioned into colors), symbols (stars in particular region), etc.

I used a hybrid metric that tries to consider colors and shapes of individual flags, and grouped countries with similar flags together.

For more technologically inclined, here's a description of technique used to group them together.