# Pool Reserves

`PoolReserves` are the fundamental building block for Neutron DEX’s AMM design. Each `PoolReserves` instance represents a single side of a liquidity pool. In addition to the `PairID`, `TokenIn` and `TickIndex` fields, Pools Reserves also have a `Reserves` and `Fee` field.

```protobuf
type PoolReserves struct {
    PairID    *PairID
    TokenIn   string
    TickIndex int64
    Reserves  sdk.Int
    Fee       uint64
}

```

`Reserves` is used to store the total amount of `TokenIn` within a given `PoolReserves` instance and `Fee` is the portion of the trading price that will be return to the pool.

In the context of LP liquidity, `PoolReserves` exist in reciprocal pairs with one side (the LowerTick) holding Token0 and the other side (the UpperTick) holding token1. Each of these pairs makes up a single constant price liquidity pool. Within each liquidity pool the following two invariants will always hold true:

1. Both PoolsReserves within a pair will have the same fee: $$LowerTick.Fee == UpperTick.Fee$$
2. The distance between the tick indexes will be equal to 2x the fee: $$LowerTick.TickIndex + 2 \* fee = UppertickTick.TickIndex$$

When swaps occur the tokens will always be added and deducted within a single liqudity pool.

When LP liquidity is deposited with a given fee and price it is added to the `TickLiquidity` instances such that the given fee is already included in the price. For example, if Alice deposits 100 TokenA and 100TokenB at price 0 (tick 0) with a fee of 1 then her liquidity will be held in tick -1 and tick 1 respectively. If Bob were to swap 50TokenA for TokenB using Alice’s liquidity his exchange rate would be \~ .999. His 50 TokenA would be deposited into tick -1 and he would receive 49 TokenB which would be deducted from tick 1.

It is important to note that multiple `PoolReserves` can exist with the same TickIndex but each one will have a unique fee.