When you observe a marching band formation that appears to take on the shape of a letter even though the members are not in direct contact with each other, you are experiencing the gestalt grouping principle called

When you observe a marching band formation that appears to take on the shape of a letter even though the members are not in direct contact with each other, you are experiencing the gestalt grouping principle called closure or continuity
or common fate.
They all denote the perceptual tendency to integrate disconnected pieces into a whole image.