boundary metaphor

A separator with a way through it. Wall, gate, fence, door, membrane, filter. You pour a membership rule or a blocking constraint into it: this side and that side, and the one opening that lets things across. It is the sharpest class for showing that something is not hard but closed. What it adds for free is singleness and solidity. A wall picture says there is one gate, and that the rest of the barrier is not negotiable.

A separator with a way through it. Wall, gate, fence, door, membrane, filter. You pour a membership rule or a blocking constraint into it: this side and that side, and the one opening that lets things across. It is the sharpest class for showing that something is not hard but closed. What it adds for free is singleness and solidity. A wall picture says there is one gate, and that the rest of the barrier is not negotiable.

Example

Best when the content is a clean in-or-out rule with a specific opening you want the reader to look at.