Sufficient and Necessary Conditions on the LSAT: How to Stop Reversing the Rule

By AccessPrep Editorial · Updated November 6, 2026 · Facts checked November 6, 2026

The workshop, choir and seed library below are fictional, and every rule, review note and practice item is AccessPrep-created: not an official LSAC question, not a real student’s record. AccessPrep and Lovare share a founder, and Lovare offers paid LSAT instruction and admissions consulting.

A conditional rule runs one way. The sufficient condition guarantees the necessary one; the necessary condition is required, but this one-way rule alone does not make it sufficient. From a rule written as S → N, two conclusions are safe: when S happens, N happens, and when N fails, S fails (the contrapositive). Two tempting moves are invalid: concluding S because N happened (reversal), and concluding that N failed because S failed (negation). To test a conditional claim, try to describe one concrete situation where the rule is true and the claim is false; if you can, the claim does not follow. Many reversals start in translation, so learn where “only if”, “unless”, “requires”, “all”, “only” and “no” put the arrow.

Jump to: Conditions · Translation · Four moves · Counterexamples · Worksheet · Practice

Who this guide is for

You know the words “sufficient” and “necessary”, yet blind review keeps turning up answers that flip a rule, or arguments you accepted because they looked like a contrapositive. Success means that, given an unfamiliar rule, you can write its arrow, name the move each answer choice makes, and prove every rejection with a situation.

The worked lesson in AccessPrep’s blind-review guide takes an “only if” rule and an “only” rule through a reversal, one counterexample and a contrapositive. This guide builds on that counterexample with new examples: the wordings that hide the arrow, negation as a separate trap, and a counterexample test for any answer choice.

LSAC includes applying rules among the skills assessed by Logical Reasoning, while explaining that specialized logical terminology is not required. The labels here help you inspect reasoning; memorizing labels is not the goal.

What each condition does

Take a rule from the Kestrel Street Workshop, a fictional community workshop:

The laser cutter is used only if a staff technician is on site.

Laser use is sufficient: learning the cutter was used is enough to know a technician was there. A technician on site is necessary: the cutter is never used without one, but a technician’s presence does not make anyone use it. Write the arrow from sufficient to necessary: Laser used → Technician on site. In typed notes, -> works the same.

A one-way rule bans exactly one situation and says nothing about the other three:

Every valid inference comes from crossing out that one cell; both invalid moves below also cross out the cell people forget.

Translate the wording before you infer

Copy this mapping table

Three readings need extra care.

“Only” versus “all.” “Only fob holders enter after hours” means everyone who enters holds a fob, not that every fob holder enters. The term right after “only” is necessary; the term right after “all” is sufficient. Read the whole sentence rather than treating individual words as automatic switches: in “the only people who enter after hours are fob holders”, entering is sufficient.

“Unless.” This guide uses the standard logic reading: “A unless B” means “if not B, then A”, so the workshop closes at 9 p.m. if no evening class runs. On class nights the rule is silent: the class can end at 8:30 and the workshop still close at 9. The logic textbook forall x: Calgary (section 5.6, “Unless”) gives the same reading and warns that everyday speakers often mean both directions. LSAC’s Logical Reasoning page does not define “unless”, so treat this as a logic convention rather than a rule LSAC states; the “does not follow” verdicts on this guide’s “unless” items depend on it.

“No.” Treated → Not in booth and In booth → Not treated are the same rule, so swapping the terms is safe. Dropping the “not” is the trap: “anything that is not treated lumber goes in the booth” does not follow.

A rule stated with “if and only if” runs both ways. This guide concerns one-way rules.

Four moves: two valid, two invalid

For a single rule and a fact establishing or denying one condition, these four moves cover the basic cases. More complex arguments may combine rules or add other premises.

Use it in order: write the arrow, then find the row for the fact you were given. Only the first two rows license a conclusion; in the last two, this rule alone settles nothing about the other condition.

Reversal and negation fail together. They look different, but each is the contrapositive of the other, so one situation (necessary present, sufficient absent) breaks both; start there when you hunt for a counterexample.

The counterexample test, worked

A claim follows from a rule only if no situation makes the rule true and the claim false. So try to build that situation.

Reversal claim: “A technician was on site Wednesday, so the laser cutter was used Wednesday.”

Arrow: Laser used → Technician on site. Situation: on Wednesday a technician counts stock all day, and nobody uses the cutter. The rule holds, because only laser use without a technician breaks it, and the claim is false. One such situation is enough: the claim does not follow.

Negation claim: “The cutter was not used Thursday, so no technician was on site Thursday.” Give Thursday Wednesday’s facts: the rule holds and the claim is false.

Contrapositive claim: “No technician was on site Sunday, so the cutter was not used Sunday.” Making this false requires the cutter to run with no technician, the one situation the rule forbids, so no counterexample exists and the claim follows.

Not finding a counterexample is not enough to prove validity. For these two-condition examples, inspect all four rows of the possibility table. For a chain, derive each step from a stated rule. If you can do neither, record UNRESOLVED rather than marking the claim valid.

Two limits: a counterexample shows only that the claim does not follow from this rule (other premises might still establish it), and it must keep every stated fact true.

Denying a rule. The denial of S → N is not S → not N; it is the forbidden cell: S happened and N did not. One window piece from another class falsifies “every piece in the front window came from the beginners’ class”. “No window piece came from the beginners’ class” does not deny it: one beginners’ piece beside one advanced piece makes both false.

A mistake and its correction

AccessPrep-created argument and fictional review note, not a real student’s log: “The paint booth is used only if the exhaust fan is on. The paint booth was not used yesterday. So the exhaust fan was off yesterday.”

Original note: “Valid. Both parts got a ‘not’, so it’s the contrapositive.”

What went wrong: the arrow is Booth used → Fan on, so the contrapositive is Fan off → Booth not used. The argument added “not” to both conditions but never swapped them. That is a negation.

Counterexample: the fan ran during a maintenance check and nobody painted. The rule holds, and the conclusion is false.

Corrected note: “Both sides having ‘not’ is not enough. Before calling anything a contrapositive, check that the condition after the arrow moved to the front.”

Copy this counterexample worksheet

Use one card per claim, including claims from questions you missed.

A situation counts only if both checks say yes and the given fact stays true.

Free packet: Download the worksheet, worked lesson and truth-table checks. No email required.

Transfer exercise

Every item is an AccessPrep-created practice item, not an official LSAC question. Answer, with a counterexample or “none possible”, before reading the explanations.

Part A. The Linden Community Choir is fictional. Does each claim follow from the rule and the fact alone?

1. Rule: The choir rehearses in the main hall unless the hall is booked for a wedding. Fact: The hall is booked for a wedding this Saturday. Claim: The choir does not rehearse there this Saturday.

2. Rule: Singing a solo at the spring concert requires attending its dress rehearsal. Fact: A tenor missed that dress rehearsal. Claim: The tenor does not sing a solo at the spring concert.

3. Rule: Only altos sit in the front row. Fact: A singer is an alto. Claim: The singer sits in the front row.

4. Rule: Whenever the director is absent, the accompanist leads warm-ups. Fact: The director is absent Thursday. Claim: The accompanist leads warm-ups Thursday.

5. Rule: All singers in the chamber group also sing in the full choir. Fact: A soprano is not in the chamber group. Claim: The soprano does not sing in the full choir.

6. Rule: No singer who missed two rehearsals in a row this month performs at the next concert. Fact: A bass performs at the next concert. Claim: The bass did not miss two rehearsals in a row this month.

Part B. AccessPrep-created practice item, not an official LSAC question. The Alder Valley Seed Library is fictional.

A seed packet is on the library’s public shelf only if it passed this year’s germination test. No packet of hybrid seed passed this year’s germination test. Every packet donated by the county farm is on the public shelf.

If the statements above are true, which one of the following must also be true?

(A) Every packet that passed this year’s germination test is on the public shelf.

(B) No packet donated by the county farm is hybrid seed.

(C) Every packet that is not hybrid seed passed this year’s germination test.

(D) Every packet that passed this year’s germination test was donated by the county farm.

(E) Every packet not donated by the county farm is off the public shelf.

Answers and explanations

1. Does not follow (negation). Hall not booked → Main hall. “Booked” means the sufficient condition failed. Counterexample: the wedding starts at 6 p.m. and the choir rehearses in the main hall at 10 a.m.; the rule promises the main hall only on unbooked days. (Under a two-way reading of “unless” the claim would follow; this item tests the convention above.)

2. Follows (contrapositive). Solo at the concert → Dress rehearsal. The necessary condition failed, so no solo at that concert. A counterexample would need a soloist who skipped the rehearsal, which the rule forbids.

3. Does not follow (reversal). “Only altos” makes alto necessary: Front row → Alto. Counterexample: an alto who sits in the second row, while every front-row singer is still an alto.

4. Follows (apply the rule). Director absent → Accompanist leads, and the fact is the sufficient condition.

5. Does not follow (negation). Chamber group → Full choir. Counterexample: a soprano who sings only in the full choir.

6. Follows (contrapositive). Missed two in a row this month → Does not perform at the next concert. Performing means the necessary condition failed, so the bass did not miss two in a row this month.

Part B: (B). Translate: Shelf → Passed; Hybrid → Not passed, which equals Passed → Not hybrid; Farm → Shelf. Link rules only where one rule’s necessary condition is the next rule’s sufficient condition: Farm → Shelf → Passed → Not hybrid. So no farm packet is hybrid seed.

(A) reverses Shelf → Passed: a home gardener’s non-hybrid packet could pass and still sit in storage. (C) negates Hybrid → Not passed: a non-hybrid packet that failed the test breaks it. (D) reverses the chain Farm → Passed: a home gardener’s packet that passed and is on the shelf breaks it. (E) negates Farm → Shelf: that same shelved packet breaks it.

See the whole chain before reversing anything

For the seed-library exercise, connect the same packet through four conditions:

1. County-farm donation implies public shelf.

2. Public shelf implies passed germination test.

3. Passed test implies not hybrid, the contrapositive of hybrid implies not passed.

So Farm implies Shelf implies Passed implies Not hybrid. Follow the arrows in that direction. You may also reverse the entire chain while negating every condition: Hybrid implies Not passed implies Not shelf implies Not farm. Simply reading the original chain backward is invalid.

Use one allowed packet to challenge several attractive reversals: a non-hybrid packet from a home gardener passes and is placed on the shelf. All three original rules hold, yet “passed implies farm” and “not farm implies not shelf” are false. One valid counterexample is enough to disprove a must-be-true claim; it need not be the most likely situation.

Troubleshooting

The necessary condition has “and” in it. If booking the kiln requires a signed form and a firing class, missing either one rules out a booking: the contrapositive of “S → A and B” is “not A or not B → not S”. Having both still does not show a booking.

You translate correctly in review but flip rules under time. Try writing the arrow before reading the choices when a rule contains “only”, “unless” or “requires”, and run the counterexample test on your last two contenders. Log translation errors (arrow backward) apart from move errors (arrow right, conclusion wrong): the first need the mapping table, the second the worksheet.

The wrong answer seems true in real life. A must-be-true answer has to hold in every situation the rule allows, not just the likely ones.

When to stop drilling conditionals

Build each drill set from four rules in different wording families, taken from official questions you have already reviewed: write all four moves for each rule on separate cards (16 claims), shuffle, and mark them the next day. Stop isolated drills after two sets in a row, on different days, with zero direction errors and a written counterexample for every rejected claim. A direction error is any reversal, negation or backward arrow; one restarts the count.

Then move to full Logical Reasoning questions from official LSAT practice and log every conditional miss. If a direction error appears in two separate sessions, return to the worksheet for that wording family only. These thresholds are an AccessPrep teaching proposal, not a validated cutoff, and meeting them does not predict a score.

Put the counterexample test to work.

Free first step: run your last three missed conditional questions through the worksheet card, using the paint-booth correction above as your model, and label each error translation or move. Then take the free LSAT Study-Plan Check: a free way to organize your next study priorities, with no email required.

Optional paid path: current one-on-one LSAT programs are offered through Lovare Institut, which shares a founder with AccessPrep. The Standard and Elite private options currently listed each include 10 tutoring hours (checked November 6, 2026), so work through the worksheet and the stopping rule first. If direction errors persist alongside other problems, explore one-on-one LSAT support or contact AccessPrep to discuss your plan.

Continue your preparation

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