Freestyle · Pattern
Open three.
Three of your stones connected in a straight line, with both ends empty, is an open three. Because both ends are open, the line can grow in two directions — that is what makes it a threat.
G9–H9–I9: both ends (F9 and J9) are empty.
Try it: choose one open end
This page only demonstrates straight, contiguous open threes. Broken and jump patterns are separate ideas and are not covered here.