> For the complete documentation index, see [llms.txt](https://docs.leather.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.leather.io/learn/what-is-stacks.md).

# What is the Stacks Bitcoin L2?

Stacks is a Bitcoin layer for smart contracts. Its token, its smart contract language, its consensus and what runs on it.

Stacks is an open-source blockchain that adds smart contracts and apps to Bitcoin. It has its own token, STX, its own smart contract language, Clarity, and a consensus mechanism tied to Bitcoin, Proof of Transfer.

## Clarity

Clarity is the language of Stacks smart contracts. A contract's source code is published on chain and runs as written; it is not compiled. Clarity contracts can read the state of Bitcoin, so they can react to Bitcoin transactions.

## Proof of Transfer

* **Miners** spend BTC for the right to produce Stacks blocks, and earn new STX and fees.
* **Stakers** lock STX with a pool and earn rewards in sBTC, funded by the BTC miners spend.
* **Signers**, chosen through staked STX, approve each block. A block needs signers holding 70 percent of the signing weight.

See [What Is Proof-of-Transfer (PoX)?](/learn/proof-of-transfer-pox.md).

## Fast blocks, tied to Bitcoin

Since the Nakamoto upgrade, which activated on mainnet in October 2024, Stacks produces blocks every few seconds. Each group of Stacks blocks, a tenure, is tied to a Bitcoin block. Once that Bitcoin block is confirmed, reversing those Stacks blocks would require reversing Bitcoin.

## sBTC

sBTC is a token on Stacks backed 1:1 by BTC. It brings bitcoin into Stacks apps, and staking rewards are paid in it. See [sBTC](/stacking-and-sbtc-reference/sbtc.md).

## PoX-5

PoX-5 activated at Bitcoin block 960,230 on July 30, 2026. It replaced Stacking with STX-only staking to signer-managers, added Bitcoin Staking with protocol bonds, and pays rewards in sBTC. See [Getting started with STX-only staking](/stacking/getting-started-with-stacking.md).

## What runs on Stacks

Exchanges, lending protocols, stablecoins, NFT marketplaces, name services such as BNS and staking services. Leather holds STX and Stacks tokens and NFTs, connects to Stacks apps and stakes STX. To get STX, see [Buy STX](/get-started/buy-stx.md).