Why does software development slow down over time? One of the many reasons is that the existing code places constraints on your ability to go forward. You must keep all prior features working while adding the next.

The interactive activity below demonstrates this phenomenon.

Your Goal

Fill the grid above with the numbers 1 through 9 such that:

  • Every cell has a unique number


To fill in the grid, type the desired numbers on your keyboard. You can also move the selection around using the enter or arrow keys.

Why Constraints Slow You Down

Hopefully the activity above helps you intuit why adding constraints slows you down. At a superficial level, the work appears to be the same throughout: you are "only" putting nine numbers in a grid. But each successive constraint restricts the placement of the numbers on the grid, changing the nature of the work and the effort required as you progress.

When the only constraint is to place the nine unique numbers, the work is trivial. Place them in any order you like. One of the simplest methods is to just do them in sequential order. But as the other constraints enter the picture, you have to change your approach to placing the numbers. You may have to significantly rearrange the numbers on the grid to move them into positions that make the sums work out. Even the initial work of making sure you have all nine unique numbers becomes harder as you ask yourself, "Which number am I missing now that I've moved some?"

The same thing happens in software development. At a superficial level, the work all seems the same. Why are some things more difficult than others when it all appears to be "only" writing some code? The answer is that there are hidden constraints. Setting aside the inherent technology constraints, every feature you build also acts as a constraint. Adding new features needs to not break old features, and that leads to more mental effort and longer development times just as illustrated in our activity.


This page is based on an activity I experienced as part of a mob programming workshop which Woody Zuill presented. The participants drew a hash (the '#' symbol like a tic-tac-toe board) on paper then filled in the numbers. We worked in pairs, verifying that each other's numbers satisfied all the constraints. The entire room worked at the same time, and Woody timed how long it took for the pairs to finish.

It is a simple exercise to repeat in a group if you have access to writing materials. You can use these example constraints as illustrated on the right:

  1. Each space must be filled with a unique number from 1 though 9
  2. The right side must sum to 15
  3. The top must sum to 9
  4. The diagonal must sum to 18

The facilitator should make sure to inquire why the successive rounds are taking longer, making sure to explain the concept at the end.