when and only when

USAGE SUMMARY

The phrase "when and only when" is correct and usable in written English.
You can use it when you want to emphasize that a certain condition is essential for something to take place. For example, "The project will be a success when and only when everyone works together."

✓ Grammatically correct

Science

News & Media

Encyclopedias

Human-verified examples from authoritative sources

Exact Expressions

59 human-written examples

According to this view, the human mind is like an eye that can see when and only when God, the source of light, illumines it.

Encyclopedia Britannica

Formulas so formed, however, are valid when and only when they are substitution-instances of valid wffs of PC and hence in a sense do not transcend PC.

Encyclopedia Britannica

"In the majority of cases, these losses occurred when, and only when, people began to expand across areas that had never before experienced their presence".

The New Yorker

The Crown Prosecution Service is committed to bringing the right charges to court when – and only when – it is proper to do so".

The Guardian

And that is when, and only when, measures should be discussed both to deter the perpetrators and ultimately arrest and try them".

The Guardian

Her Majesty's exploited servants were to show tourists round Buckingham Palace's staterooms, when and only when the monarchy thought it had no choice but to pay them.

The Guardian - Opinion

We can only hope to recognize our mistakes and trust that our government will call for national sacrifice when, and only when, the circumstances require it, and when, and only when, we are responding to real threats and not, as was true with Iraq, for the sake of flexing our military might against imagined enemies.

The New York Times

For example, they might discover that a certain chemical is released when and only when people are frightened or that a certain region of the brain is activated when and only when people are in pain or think of their fathers.

Encyclopedia Britannica

And in America, at least, we have a pretty good record for behaving in a fiscally responsible fashion, with one exception — namely, the fiscal irresponsibility that prevails when, and only when, hard-line conservatives are in power.

The New York Times

In the first category, Mr. Feldman and Mr. Goode assert that smarter foreign policy can be found through improved policy making — presidential deference to Congress; acting "when, and only when," certain metrics are met; and so on.

The New York Times

For multilayer-SPPs, the horizontal asymptote appears when and only when β goes to infinity.

Nano-Micro Letters

Expert writing Tips

Best practice

Use "when and only when" to establish a clear, bidirectional relationship between two events or conditions. This signals that one occurs if, and exclusively if, the other does.

Common error

Avoid using "when and only when" if you only mean 'when'. The phrase indicates a strict equivalence; if the second part doesn't guarantee the first, it's inappropriate.

Linguistic Context

The phrase "when and only when" functions as a conjunctive adverb, establishing a necessary and sufficient condition between two clauses. It specifies a strict bidirectional relationship, meaning that one event occurs if, and exclusively if, the other does. Ludwig confirms its proper usage.

Expression frequency: Very common

Frequent in

Science

35%

News & Media

25%

Encyclopedias

15%

Less common in

Formal & Business

10%

Reference

5%

Wiki

10%

Ludwig's WRAP-UP

In summary, "when and only when" is a grammatically correct phrase used to establish a strict, bidirectional relationship between two conditions or events. According to Ludwig, the phrase is perfectly acceptable for use in writing. It's commonly found in scientific, news, and encyclopedia sources, indicating its suitability for both formal and neutral contexts. Although some related phrases offer similar conditional meanings, "when and only when" emphasizes a specific, necessary, and sufficient relationship. It's vital to use it accurately, ensuring the bidirectional condition truly applies, to avoid misrepresenting the intended meaning. The phrase is useful for scenarios needing precise communication.

More alternative expressions

Phrases that express similar concepts, ordered by semantic similarity:

if and only if

Slightly more formal and mathematically precise.

precisely when

Emphasizes accuracy and exactness in timing or condition.

exclusively when

Highlights the exclusiveness of the condition.

specifically when

Highlights a particular time or set of circumstances.

solely under the condition that

Emphasizes the singular condition required.

just in the event that

Adds a sense of contingency or possibility to the condition.

dependent upon the condition that

Highlights dependency on the specific condition being met.

only in circumstances where

Specifies that the condition is circumstance-dependent.

contingent on the condition that

Emphasizes that it will happen depending on one specific case.

not unless and until

Emphasizes the negative and temporal aspect of the condition.

FAQs

How can I use "when and only when" in a sentence?

Use "when and only when" to indicate that something occurs if and only if a specific condition is met. For example, "The system will activate "when and only when" the correct password is entered."

Is there a shorter way to say "when and only when"?

Yes, the phrase "if and only if" is a shorter, mathematically precise equivalent to "when and only when". Both establish a necessary and sufficient condition.

What is the difference between "when" and "when and only when"?

"When" indicates that something happens under a certain condition, but that condition may not be the only one. "When and only when", on the other hand, specifies that the condition is both necessary and sufficient for the event to occur.

Can I use "when and only when" in informal writing?

While "when and only when" is grammatically correct, it can sound somewhat formal. In informal writing, consider using simpler alternatives if the strict bidirectional meaning isn't crucial. The alternative "only if" can be used, yet it has a slightly different meaning.