# Bitcoin Concepts

**What is OP\_CAT and why does it matter?**

OP\_CAT is a re-enabled Bitcoin opcode that allows concatenation of data in script. On Fractal, OP\_CAT powers secure smart contracts, recursive covenants, and modular logic across Bitcoin-native assets.

[→ Learn about OP\_CAT](https://fractalbitcoin.io/learn/what-is-op-cat)

**What are Bitcoin covenants?**

Covenants let you restrict how UTXOs can be spent — enabling things like vaults, decentralized governance, or time-locked contracts. Fractal supports expressive covenants through OP\_CAT.

[→ Learn more](https://fractalbitcoin.io/learn/bitcoin-covenants)

**What’s the difference between CAT20, BRC-20, and Runes?**

* **CAT20**: Contract-enabled token standard native to Fractal, designed for programmable logic using Bitcoin Script
* **BRC-20**: Simple inscription-based token format that originated on Bitcoin, now supported natively on Fractal
* **Runes**: Lightweight, inscription-based token protocol supported on both Bitcoin and Fractal

All three standards are **natively supported** on Fractal.

[→ Compare token standards](https://fractalbitcoin.io/learn/understanding-cat-protocol)