Blocks & the 10-Minute Heartbeat

Hook

Unconfirmed transactions wait in the mempool. Settlement happens when a batch of them is written into a block—and blocks arrive on a slow, steady rhythm, not a card-network blink.

One idea

A block is a batch of transactions plus a link to the previous block; the chain of those links is Bitcoin’s ordered history, and new blocks arrive about every ten minutes on average—the network’s heartbeat.

Plain walkthrough

Batches, not one-by-one finality. When you broadcast a send, nodes share it and it sits unconfirmed until a miner includes it in a candidate block (Chapter 12). A block packages many transactions together—whoever paid competitive fee-rates tends to fit when space is tight—along with metadata that ties this batch to the one before it.

The chain links history. Each block refers to the previous block’s identity (a cryptographic fingerprint called a hash). That pointer makes a chain: rewrite an old block and every later link breaks. “The blockchain” in everyday talk is this shared, append-only sequence of batches. Newer blocks bury older ones deeper; each extra block on top is another confirmation for the transactions inside (Chapter 9).

About ten minutes—on average. The protocol aims for a new block roughly every ten minutes. Sometimes two come quicker; sometimes you wait longer. The average is a design choice: slow enough for the network to propagate and agree, fast enough to be usable. It is not a promise that your transaction lands in the very next beat—that still depends on fees and space.

What “in a block” means for you. Your wallet flips from pending to confirmed when it sees your transaction in a block it accepts. Merchants and careful users often wait for more confirmations before treating large value as settled. Depth in the chain is what makes reversal increasingly hard—not a customer-service undo desk.

Hero viz (optional). Viz’s Build the Next Block playground (/playground/build-the-next-block) — “Pack the next block” — is the hands-on hero for this stretch: scarce space, best fee-per-space wins. Use it when you want muscle memory; this chapter teaches the idea without requiring the sim. Pure sim at ship—no live mempool dependency required.

A short picture after the mechanism. Imagine a bound notebook: every ~ten minutes someone adds a page of stamped receipts and writes the previous page’s seal at the top. Tear out an old page and the seals no longer match. The mempool is the stack of receipts still on the counter; the notebook is the chain.

Blocks connect forward. Who gets to propose the next page, and why they spend energy to try, is Chapter 14. That miners propose pages does not mean they invent the rules—nodes check every page (Chapter 15).

Watch-outs

• “Ten minutes” is an average — Your wait can be shorter or longer; congestion and fee-rate matter.
• Broadcast ≠ confirmed — Hearing about a transaction is not the same as inclusion in a block.
• One confirmation ≠ maximum caution — More depth usually means more practical finality; counterparties set their own thresholds. We do not prescribe a fixed “safe N.”
• Explorers show many numbers — Block height, hashes, and merkle details are normal. For learning, focus on: is my tx in a block, and how many blocks sit above it?
• Playground ≠ mainnet — Build the Next Block does not mine real bitcoin or settle real payments.

You now can…

• Describe a block as a batch of transactions linked to the previous block.
• Treat ~10 minutes as the heartbeat average, not a personal SLA.
• Connect confirmations to depth in the chain—and know the fee waiting room (Ch 12) feeds that heartbeat.