Companies stop shipping the moment ownership goes soft. A goal everyone is responsible for is a goal nobody is responsible for, because responsibility without singular ownership is just a feeling. The single-thread principle is the discipline of assigning exactly one person to every outcome that matters, then giving them the room and rope to deliver it.
Why the rule keeps breaking
The trap is collaboration. It feels generous to say "we will own this together," but committees do not own things, they coordinate them. Coordination is overhead, not progress. The instant an outcome is collective, decisions migrate to the slowest reviewer in the room and the work stalls in inboxes.
What single-threading actually looks like
The owner is named, in writing, with the outcome they are responsible for and the date it is due. They have the authority to make the trade-offs the work demands, and they are the one accountable when it lands or does not. Other people contribute, but only one name sits next to the outcome.
Where it earns its keep
Cross-team initiatives, anything spanning product and operations, new market launches, and any change that requires a sequence of small decisions. The faster the cadence of decisions, the more dangerous shared ownership becomes.