Many stablecoin protocols have entirely embraced one spectrum of design (entirely collateralized) or the other extreme (entirely algorithmic with no backing). Collateralized stablecoins either have custodial risk or require on-chain overcollateralization. These designs provide a stablecoin with a fairly tight peg with higher confidence than purely algorithmic designs. Purely algorithmic designs such as Basis, Empty Set Dollar, and Seigniorage Shares provide a highly trustless and scalable model that captures the early Bitcoin vision of decentralized money but with useful stability. The issue with algorithmic designs is that they are difficult to bootstrap, slow to grow (as of Q4 2020 none have significant traction), and exhibit extreme periods of volatility which erodes confidence in their usefulness as actual stablecoins. They are mainly seen as a game/experiment than a serious alternative to collateralized stablecoins.
Frax attempts to be the first stablecoin protocol to implement design principles of both to create a highly scalable, trustless, extremely stable, and ideologically pure on-chain money. The Frax protocol is a two token system encompassing a stablecoin, Frax (FRAX), and a governance token, Frax Shares (FXS). The protocol also has a pool contract which holds USDC collateral. Pools can be added or removed with governance.