# The secret of the 7th row - visually explained | Summary and Q&A

429.2K views
January 26, 2019
by
Mathologer
The secret of the 7th row - visually explained

## TL;DR

Circle stacking follows fascinating patterns and symmetries, with row seven always being perfectly level.

## Key Insights

• 🟤 Circle stacking involves rows becoming crooked when the circles in the first row are not equally spaced.
• 🔚 Row seven always ends up being level, regardless of the arrangement of the circles in the first row.
• 💁 The tip of the pyramid in a circle stack is always exactly centered between the two walls, regardless of the shape of the stack.
• ⭕ Circle stacking exhibits half-turn symmetry, with the red circle at the top becoming the center of this symmetry.
• 🟤 The internal structure of a circle stack determines the levelness and alignment of the rows.
• 😜 An ill-formed stack can be a phase transition between well-behaved stacks with different numbers of circles at the bottom.
• 👻 Circle stacking phenomena can be visually demonstrated through physical models, allowing for a better understanding of its underlying principles.

## Transcript

Welcome to another Mathologer video. Have a look at this -- four equally spaced circles in a box. Let's fill up the box with circles by stacking them row by row like this: stack, stack, stack. Okay let's start again and have a closer look at what's happening here. Because the first row of circles is perfectly level and the circles are equally space... Read More

### Q: Why does row 7 always end up being level in circle stacking?

Row 7 remains level because of the symmetric structure of the stack, ensuring that the corners in the rhombus cells are vertically aligned, resulting in a level row. The arithmetic behind it can be understood by examining the parallel lines and their alignments.

### Q: How does the grid underlying a circle stack exhibit half-turn symmetry?

The grid exhibits half-turn symmetry because it can be constructed by combining a super grid with a copy of it turned 180 degrees. This symmetry extends to the associated stack, with the red circle at the top being the center of this symmetry.

### Q: Can circle stacking be applied practically?

While no practical applications of circle stacking are currently known, the concept of circle stacking and its underlying principles could potentially inspire new ideas in architecture, design, and structural engineering.

### Q: Are there any variations in circle stacking, such as using different-sized circles or non-vertical walls?

Circle stacking can still exhibit interesting patterns and aligning properties even when using alternating sizes of circles for different rows or when the walls of the box are not perfectly vertical. The alignment of critical rows and the overall symmetry still hold true.

## Summary & Key Takeaways

• In circle stacking, when circles are stacked row by row, the rows become crooked when the circles in the first row are not equally spaced.

• However, row seven always ends up being perfectly level, regardless of the arrangement of the circles in the first row.

• Circle stacking exhibits various phenomena, including level rows, half-turn symmetries, and alignment of the tip of the pyramid with the center of the walls.