---
url: /api/kiln/functions/Mut.md
---
[Kiln API](../../index.md) / [kiln](../index.md) / Mut

# Function: Mut()

```ts
function Mut<T>(trait): MutParam<T>;
```

Marks a trait as written by this system. Every other trait in the query arrives read-only,
and the scheduler uses Mut() as the system's write set.

## Type Parameters

| Type Parameter |
| ------ |
| `T` *extends* `Trait` |

## Parameters

| Parameter | Type |
| ------ | ------ |
| `trait` | `T` |

## Returns

[`MutParam`](../interfaces/MutParam.md)<`T`>