When you meld a run, the two cards at its ends become gold to every opponent. Throwing a card next to that run hands them an extension they can dump for zero cost.
Once you meld a run like 5♠-6♠-7♠, the run sits on the table for the rest of the round. Any player, including your opponents, can extend it. The 4♠ extends the low end. The 8♠ extends the high end.
Adjacent means the two ranks immediately next to your run's ends, in the same suit. For 5♠-6♠-7♠, that's the 4♠ and 8♠.
This tip says, if you have a 4♠ or 8♠ in your hand, do not throw it. Any opponent will pick it up (or dump their own copy onto your meld) and get rid of a card for free.
On Play Rummy, extensions are a single drag. There's no cost to an opponent that extends. Which is why this tip exists.
You already scored the meld. Extending doesn't lose you points directly. So why is it bad?
Because the opponent just shed a card. That card would have counted against them at the end of the round. Now it doesn't. They saved a few points (up to ten if the card was a face card).
Every point they save is a point you effectively lost. Rummy is a differential game. The gap between your score and theirs is what matters. Extension is a way for them to close the gap without doing anything hard.
Do this before every discard when you have melds on the table.
Step one, list your table runs. Note the suit and the end ranks.
Step two, note the adjacent ranks. For each run, one rank below the lowest card and one rank above the highest card.
Step three, check your hand. If you have any adjacent card, keep it. Discard something else.
Three steps. Two seconds. The scan becomes automatic once you've done it a few dozen times.
Holding the adjacent card has costs. It's still a card in your hand at end of round if you don't place it.
Two better outcomes for it.
Extend your own meld with it. If your 5♠-6♠-7♠ is on the table and you have the 8♠, put the 8♠ on your meld yourself. You take zero point cost and no opponent gets the extension.
Use it for a different meld. If the 4♠ pairs with your 4♥ toward a set of fours, keep working on that set. The 4♠ has a home in your hand.
If neither of those works, hold it as long as you can and dump it late when the risk is smaller (or when the round is ending anyway).
Runs have adjacency (the ranks next door). Sets have a different form of extension. A meld of three eights (8♠-8♥-8♦) can be extended by any 8 of the fourth suit (8♣).
So the same rule applies. If you meld a set of eights, don't throw the fourth eight. Any opponent will dump it for free.
Sets are usually less dangerous than runs because only one card in the whole deck extends them (the last suit of that rank). Runs have two adjacencies. But the principle is the same. Don't hand free rides.
Sometimes the adjacent card is already accounted for. If both copies of 4♠ are in the discard pile (or one is in an opponent's melded run), the 4♠ from your hand is safe to throw. No opponent has one to gift-dump.
Similarly, if you've already extended your own run with the 8♠, the 4♠ side is now the only adjacency. And if the 4♠ is in another opponent's hand (they picked it up), it doesn't matter what you throw. They already have theirs.
Track your table melds' adjacencies over time. Sometimes the danger goes away without you doing anything.
Expert players think about adjacency before they meld, not just after.
A three-card middle run like 5-6-7 has two adjacency cards. A three-card end run like J-Q-K has one adjacency card (only the 10 side). Same-suit end runs are safer to lay down because they leave fewer extensions available.
Same idea, a run of A-2-3 has one adjacency card (the 4 side, assuming Ace low). Also safer.
Middle runs give more extensions to opponents. If you have a choice between melding a middle run or an end run first, meld the end run when possible. It exposes less.
If you can grow your own run to four or five cards, do it. Every card you extend it with is one less adjacency for opponents.
Your run of 5-6-7 has two adjacencies (4 and 8). If you extend to 5-6-7-8, only one adjacency remains (4). If you extend to 4-5-6-7-8, both adjacencies are gone (only 3 and 9 remain, and those are two ranks away, less connected).
Long runs are hard to extend against. Short runs are easy. This is one more reason to build long runs when possible.
Throwing the adjacent card because it's "high." The 8♠ next to your 5-6-7 is a face-worthy card. It's tempting to dump. But every opponent will pounce on it. Fix, use it to extend your own run instead.
Forgetting the check after melding. The rule kicks in the second you meld a run. Some players make the meld and immediately discard a card they wouldn't have if they'd thought about it. Fix, always redo the scan after melding.
Applying the rule to sets sloppily. Only the fourth suit of your set is dangerous, not just any card of that rank. Fix, focus on the one specific card.
Never melding runs to avoid this trap. Some players hoard runs in hand because "melding is risky." That's worse. Meld and manage the adjacency. Hoarding costs more in deadwood.
Over your next five games, after every meld, name the adjacencies out loud (or in your head). Repeat before every discard for the rest of the round.
You'll probably catch two or three near-mistakes. Cards you were about to throw that would have gifted an opponent. Every catch is worth a few points saved.
The habit forms fast. Within a week, you'll notice adjacency risks without any conscious effort.
Every table meld has an edge. The edge is where opponents can extend for free. Guard the edge by not throwing the neighbouring rank.
This tip costs you nothing. It saves opponents from turning your melds into their dump zone.
Meld freely. Just watch the edges.