Partial runs have two ends. Sometimes you have to break one to save points or to make room. Break the end where the next card you need is mostly dead. Keep the end that still has live outs.
An "out" is any card still available in the deck (unseen, unplayed) that would complete or extend your meld. For a partial run, outs sit at the two ends.
Say you have 6♠-7♠-8♠ in hand. The two possible next cards are 5♠ (extends down) and 9♠ (extends up). Each card is a potential out.
Now count. If the 5♠ has been discarded twice already (or picked up by an opponent) it might be effectively dead. If the 9♠ hasn't been seen at all, it's fully live.
One end is dead. The other is alive. That's the shape of most run-breaking decisions.
A partial run isn't symmetric. The two ends have different survival odds.
In a fresh deck, both ends have four copies (one per suit... wait, no, only the same suit counts for a run). Both ends have exactly one card. So both start with equal outs.
But as the round plays, cards move. Some end up in the discard pile. Some end up in opponent hands. The two ends drift apart in real value.
If you don't track which end is dead, you break the wrong one. You end up holding the useless side and dropping the growing side.
You don't need to memorise every discard. You just need to know if the exact card you need has been seen.
Step one. Identify your two ends. For 6♠-7♠-8♠, that's 5♠ and 9♠.
Step two. For each end, scan your memory. Has that card been discarded? Has an opponent picked one from the pile?
Step three. The end where the answer is "seen" is dead. The end where the answer is "not seen" is live.
Simple. Two seconds if you've been paying attention. In a single-deck game there's exactly one 5♠. If it's on the pile, that end is done. Full stop.
Turn twelve. Your partial run is 6♠-7♠-8♠. You need to break it because your hand is over 30 deadwood and you need to shed points.
The 5♠ was discarded on turn three by opponent A. Then discarded again on turn eight when it cycled back (or, in single-deck, it's just gone from the moment it hit the pile).
The 9♠ hasn't been seen. It's either in the stock or in an opponent's hand.
Break the low end. Discard the 6♠. Keep 7♠-8♠ as a pair with the 9♠ still live. If you draw the 9♠, you're back to a three-card run 7♠-8♠-9♠. The break cost you 6 points (the 6♠ discard) but kept your run alive.
If you break the wrong end and discard the 8♠, you're left with 6♠-7♠ chasing the dead 5♠. That's a wasted hold.
If you're new, the default is don't break at all. Runs are hard to build. Once you have three consecutive cards, hold them.
You should only break a run when one of these is true. Your deadwood is dangerously high (over 30). You have another meld ready that needs the space. Or the round is almost over and you need to shed.
If none of those apply, keep the run intact and look somewhere else for cards to discard.
If both ends look equally alive (or you can't remember which is which), fall back on point value. Break the higher-point end.
For 6♠-7♠-8♠, the 8♠ is 8 points and the 6♠ is 6 points. Breaking by discarding the 8♠ costs 2 more points if the round ends immediately. Not a big deal.
But for J♥-Q♥-K♥, the K♥ is 10 and the J♥ is 10 (all face cards are 10). Both ends are equal in points. In that case, look at which extension card is more likely to come. K♥ needs no upward extension (King is the top). J♥ can extend down to a 10♥. Keep the J♥ end.
Rules of thumb like this are just useful when the exact out count is unclear.
Experts add one more layer. They think about which discard the opponent wants.
If you break 6♠-7♠-8♠ by discarding the 6♠, that 6♠ goes on the pile. If the opponent is building a low spades run (like 4♠-5♠-6♠), they pick it up immediately. You just funded their meld.
If instead you break by discarding the 8♠, the opponent might be less interested (if they've shown no interest in mid-spades). But the 8♠ still lands in the pile.
Expert play combines the "out count" read with the "opponent shape" read. Sometimes the dead-end break is also the pile-safe break. Sometimes they conflict. When they conflict, favor the safer discard even if the out count says otherwise. Losing your run is a small cost. Feeding an opponent's meld is a big one.
In variants with jokers, a joker in a partial run changes the math. A joker can fill either end. So a partial run with a joker at one end is really flexible.
If you have Joker-7♠-8♠, the joker stands for either 6♠ or 9♠. In that case, break the end furthest from your other holdings. If you have a 5♠ elsewhere in hand, keep the low end so you can close 5♠-6♠(joker)-7♠. Discard the 8♠.
Read your variant's rules on joker replacement. Some variants let you swap the joker out later if you draw the real card. That flexibility affects break decisions.
Breaking without counting. Some players just pick a random end. That's the worst move. If the count is against you, half of all breaks land on the wrong side. Always count.
Assuming an unseen card is live. If the round is 20 turns in and the card hasn't shown, it's probably in an opponent's hand. That's not fully live. It's contested.
Breaking too early. Partial runs sometimes revive. Don't break just because you drew a bad card. Wait one more turn.
Ignoring points. Face cards break at 10 points each. If both ends are equally alive, break the higher-point end to save deadwood.
Play the next five rounds and note every time you break a partial run. Before you break, say out loud "outs at each end." Then decide.
After five rounds, you'll do the count in one second. That's the reflex. The wrong-end breaks stop happening.
Breaking a run is a small, unpleasant decision. Do it wrong and you lose the whole meld. Do it right and you keep half of it alive for the rest of the round.
Count outs. Break the dead end. Consider what the discard tells opponents. Keep the live end and the low-point end when in doubt.
On Play Rummy, you'll break runs a few times a session. Get the reflex now and every one of those decisions is cheap.