# Core Architecture

Foundation is modular by design, consisting of two primary layers working in unison.

***

## The Two-Layer Architecture

| Layer              | Component                   | Description                                                      |
| ------------------ | --------------------------- | ---------------------------------------------------------------- |
| **Base Layer**     | USD' (USD Prime)            | Backed by fiat stablecoins and RWAs. The shared liquidity layer. |
| **Strategy Layer** | PrimeUSD, ApolloUSD, OrdUSD | Yield strategies built on top of USD'.                           |

***

## Base Layer: USD' (USD Prime)

**USD'** is the stable base asset of the Foundation ecosystem—the shared liquidity layer that all strategies are built upon.

| Property     | Description                                   |
| ------------ | --------------------------------------------- |
| **Backing**  | Fiat stablecoins (USDC, USDT) and RWA tokens  |
| **Peg**      | Soft-pegged 1:1 to the US Dollar              |
| **Function** | Shared liquidity layer for all strategy swaps |
| **Standard** | ERC20-compatible                              |

When you move capital from one strategy to another, USD' is the internal settlement layer that enables instant swaps—without requiring external DEX trades or waiting for settlement.

***

## The Foundation Vault

*The Core Accounting Engine*

The **Vault** is the immutable ledger that holds all collateral backing USD' and maintains the master map of solvency.

### Key Properties

| Property               | Benefit                                                     |
| ---------------------- | ----------------------------------------------------------- |
| **Collateral Custody** | Holds USDC, USDT, and RWA tokens that back USD'             |
| **Asset Isolation**    | Strategy contracts cannot access collateral directly        |
| **Virtual Settlement** | Strategy swaps are ledger updates—no costly token transfers |
| **Real-Time Solvency** | Maintains global `Asset ↔ Liability` mapping at all times   |

### Collateral Types Backing USD'

* **Fiat Stablecoins:** USDC, USDT, FRAX
* **RWA Tokens:** Tokenized T-Bills, private credit

***

## Strategy Layer: Yield Strategies (ERC4626)

Strategy dollars like `PrimeUSD`, `ApolloUSD`, and `OrdUSD` are built **on top of USD'** as standardized **ERC4626 Tokenized Vaults**.

### How Strategies Work

1. **Deposit:** Users deposit `USD'` to mint strategy shares
2. **Deploy:** The strategy deploys capital to external yield venues (Aave, Orderly, Acred)
3. **Yield:** Profits flow back and increase share value
4. **Withdraw:** Burn shares to receive `USD'` plus accrued yield

### Why ERC4626?

The ERC4626 standard ensures:

* **Composability** — Strategies work across all DeFi protocols
* **Standardization** — Consistent interface for deposits, withdrawals, and share accounting
* **Transparency** — Share prices and total assets are always queryable on-chain


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