Freeze The Joint
Fix the one joint exactly; change everything either side freely; where a change needs the joint to move is the edge.
Formerly Waist
Container corner castings: one fitting, any ship, crane or lorry.
Seen in Godtfred Kirk Christiansen, LEGO brick, patented 1958
Try it on your work
Find the one joint, fitting or format through which every part of the thing you're working on must connect. Fix it exactly: size, shape, rule. Then change everything on either side of it freely. Stop when a change you want needs the joint to move; that is the edge of the design.
Fix one sentence that every version of your project's description must contain, word for word. Write three descriptions for three audiences around it. Stop at three. The sentence is the waist; if it strained, it was the wrong one.
Name the smallest agreement your collaborators must all keep: a file format, a meeting time, a handover form. Freeze it. Let every other practice vary by person. Stop when a variation breaks the agreement, and fix the variation, not the agreement.
How it has been used
The Internet is often described as having a “narrow waist.” Many kinds of networks can sit below Internet Protocol, and many applications can sit above it, because the common middle is deliberately small. Innovation does not require every layer to agree on a large master standard; it requires a minimal interface that is stable enough to connect diversity on both sides.
Where it leads
Grouped under Pull the parts apart