Bitcoin

Bitcoin is a settlement network secured by proof of work, with an issuance schedule fixed at launch in 2009 and a hard cap of 21 million BTC. It was the first system to solve double-spending without a trusted intermediary, and that single property is what everything else about it follows from.

The halving is the monetary policy

New BTC enters circulation as a block subsidy paid to miners, and that subsidy halves roughly every 210,000 blocks — about every four years. The schedule is written into the software rather than decided by anyone, so issuance is knowable decades ahead. This is the mechanism behind the 21 million cap: the sum of a halving geometric series is finite.

What proof of work actually buys

Miners expend electricity to find a valid block hash, and rewriting history means redoing that work faster than the rest of the network produces it. Security is therefore denominated in energy rather than in identity or reputation. The trade is deliberate — the network is expensive to attack precisely because it is expensive to run.

Difficulty adjustment keeps block time stable

Every 2,016 blocks the protocol retargets difficulty so blocks continue arriving about ten minutes apart regardless of how much mining power joins or leaves. This is why Bitcoin has never needed an emergency intervention to keep producing blocks: the feedback loop is automatic and has held through several collapses in mining capacity.

Where the subsidy ends

As the subsidy approaches zero, miner revenue must come from transaction fees alone. Whether a fee market can sustain adequate security is the most substantive open question about Bitcoin's long-term design, and it is a genuine one rather than a talking point. It resolves over decades, not quarters.

What Bitcoin deliberately does not do

The scripting language is intentionally limited, so complex applications live on other chains or on layers built above it. Blocks are small and confirmation is slow by design, because both choices keep the cost of running a validating node low enough that ordinary users can verify the chain themselves. Every criticism of Bitcoin's throughput is a description of a trade it made on purpose.