When you can meld two groups at once, one three-card and one four-card, don't meld both. Meld the three-card group. Meld only three cards of the four-card group. Keep the fourth card in your hand as a free extension for later.
The situation this tip covers is specific. You've just closed on two legal melds at the same time. One is exactly three cards. The other is four cards.
Example. In your hand you have 5♠, 6♠, 7♠ (a three-card run) and 9♥, 10♥, J♥, Q♥ (a four-card run). Both are legal. You want to meld.
The wrong move is to lay all seven cards on the table. The right move is to lay six. The 5♠-6♠-7♠, and the 9♥-10♥-J♥ (skip the Q♥ for now).
If you meld all seven cards, both melds close at once. Your hand shrinks. Good so far.
But now every future discard is a real discard. You've spent your best cards on the table. The cards left in hand are the leftovers. Some are singletons. Some are pairs. Discarding any of them costs the pile a card the opponent might want.
By contrast, if you keep the Q♥ in your hand, one of your next discards is basically free. You can drop the Q♥ onto your own four-card run any time you want. That's a burn without any cost.
Face cards are 10 points each. If the round ends before you can meld the Q♥ separately, holding it costs you 10 points.
But here's the key. If you meld the three-card 9♥-10♥-J♥ run on turn seven and hold the Q♥, you have most of the rest of the round to extend it. Any turn after that, you drop the Q♥ onto the run. Meld becomes four cards. Hand drops the Q♥. No cost.
You've essentially converted a "meld and discard the Q♥" into "meld and extend with the Q♥." The Q♥ never gets thrown away. The pile never sees it.
This tip is intermediate for a reason. If you're new, don't stress. Melding both groups at once is not a disaster. You lose a bit of flexibility, that's all.
The main risk of overthinking this move is that you delay melding too long and miss a chance to declare. That's a bigger loss than the small gain from holding the extension.
If you can't decide in five seconds, meld both. Move on. You can level up to holding the fourth card once the rest of your game is solid.
Once you're comfortable, invert the default. Assume you'll hold the extension unless something says otherwise.
Things that say "meld both, don't hold":
The fourth card is a face card and the round looks like it might end soon. Holding a Q♥ for two more turns is fine. Holding it for eight turns is risky. If the stock is thin, meld it now.
The fourth card is your only discard candidate. If you also need to discard this turn and the only safe discard you have is that Q♥, you can't hold it. Discard rules force your hand.
The opponent is one turn from declaring. A near-declare from an opponent kills all reserves. Dump everything you can.
Experts get more clever. They use the held Q♥ as a tempo move, not just a burn.
On the turn you want to slow the game down, extend your run with the Q♥. Opponents see your run grow from 9♥-10♥-J♥ to 9♥-10♥-J♥-Q♥. That looks aggressive. They read "this player is close to closing everything." They tighten up. They stop feeding the pile.
Meanwhile your actual hand is thinner because you shed the Q♥. And your hand's plans haven't really changed. You just used a stored card to send a signal.
This works best against tight opponents who react to visible melds. Against a loose opponent, the signal is wasted. Read the room.
Three situations flip the default.
Situation one, the fourth card is a low number. A 2 or 3 as the held card is only worth 2 or 3 points if things go wrong. That's a tiny loss. If it's a low card, meld it and move on. The reserve isn't worth the small point saving.
Situation two, you have three three-card melds instead. If everything you have is exactly three cards each, there's no extension to save. Meld all of them. The rule only applies when you actually have four cards of one group.
Situation three, the fourth card also fits another meld you're building in hand. If the Q♥ could also join a set of queens you're chasing, don't extend the run with it. Hold it for the set option. This is a different kind of reserve.
Turn six. Your hand is 5♠, 6♠, 7♠, 9♥, 10♥, J♥, Q♥, K♠, 4♣, A♦. Ten cards. You have a three-card run in spades and a four-card run in hearts ready to lay down.
Deadwood if you meld both: K♠ (10) + 4♣ (4) + A♦ (1) = 15 points.
Deadwood if you meld three-card and hold Q♥: K♠ (10) + Q♥ (10) + 4♣ (4) + A♦ (1) = 25 points. Higher by 10.
On paper, melding both looks better. So why hold the Q♥?
Because the K♠, 4♣, A♦ are all going to be discards over the next few turns anyway. If you meld both groups, your next three turns you'll be forced to discard from the K♠, 4♣, A♦, or from any card you draw. That means feeding the pile with cards opponents might want.
If you hold the Q♥, one of those three next turns you drop the Q♥ instead. It's a discard-substitute. And the K♠, 4♣, A♦ still get discarded, just spread across four turns instead of three. Net effect, one fewer risky discard hits the pile.
Holding a face card too long. A Q♥ or K♠ held for eight turns is a big loss if the round ends. Meld the reserve within three turns.
Forgetting the reserve exists. The whole point is to use it. Some players hold the Q♥ and then discard other cards for six turns before remembering. Fix, glance at the reserve every turn.
Holding a card that could also be a set. If the fourth card has another meld possibility, that's a different reserve. Don't confuse the two.
Applying the rule to three-and-three. Two three-card melds have no extension to save. Meld both. The rule only triggers when one meld has four cards.
Play the next five rounds and every time you meld more than one group at once, pause. Ask yourself, "is one of these groups four cards?" If yes, hold the fourth card and note the turn number. Then track how many turns before you extend.
You'll see the pattern quickly. Extensions usually happen within one or two turns. The reserve almost always plays.
The three-and-four sequence is a small optimisation with a real payoff. It costs almost nothing to learn. It saves you one risky discard per round on average.
Meld the three. Meld three of the four. Hold the last. Drop it later as a free burn.
Every good Rummy round is a series of small saves like this. On Play Rummy, no one flags the move for you. That's on purpose. Build the habit.