> 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/stacking-and-sbtc-reference/stacks-blockchain.md).

# Stacks blockchain

Stacks is a Bitcoin layer for smart contracts and apps, tied to Bitcoin through Proof of Transfer.

Stacks is a blockchain that adds smart contracts and apps to Bitcoin without changing Bitcoin itself.

* **Smart contracts** are written in Clarity, a language whose code is readable on chain and does not compile.
* **Proof of Transfer (PoX):** Stacks miners spend BTC to produce Stacks blocks, and each Stacks block builds on a Bitcoin block. Since PoX-5, the BTC miners spend goes to STX stakers as sBTC rewards.
* **Signers** approve each Stacks block. A block needs approval from signers holding 70 percent of the signing weight.
* **STX** is the native token. See [Stacks token (STX)](/stacking-and-sbtc-reference/stacks-token-stx.md).

Leather holds STX and Stacks tokens and NFTs, connects to Stacks apps and stakes STX. Leather does not run the Stacks blockchain.