Efficient scale

A market large enough for one profitable operator and not for two, which deters entry without any effort by the incumbent.

Pipelines, regional airports, transmission networks and small-town infrastructure all fit. The entrant's problem is that entering halves the volume and destroys the returns for both, so nobody enters.

The moat lasts exactly as long as the market stays small. Growth is the enemy: a market that doubles becomes large enough for two, and the incumbent's returns fall to the competitive level.

Slow-growing markets can be the best ones to own.