Last reviewed 22 Sept 2026 · 5 min read
The symbols
Rule 1 — the chain must be unbroken. To compare A and D you need a link at every step: with no gap.
Rule 2 — the arrows must point the same way. gives . But gives nothing between A and C.
And the strictness rule: a chain of signs gives ; if even one link in the chain is strict (), the result is strict ().
So:
| Chain | Conclusion |
|---|---|
| nothing | |
| nothing | |
The either-or case
Two conclusions are an either-or pair when all three of these hold:
- they are about the same two terms,
- neither follows on its own, and
- together they cover every case the chain allows.
The form that almost always appears: the chain proves but not which of the two it is. Then "" and "" each fail alone, and together they are certain — so the answer is either I or II follows.
The other legitimate pair is with (or with ), which exhausts every possibility whatever the chain says.
Coded inequalities
A table defines symbols, for example:
P @ QmeansP # QmeansP & QmeansP % QmeansP © Qmeans
Translate every symbol into its inequality first, write the whole chain, then apply the two rules. Never work in the symbols themselves.