How do you organize C# code in to files?

// ------ C.cs

public partial class C : IFoo
{
    // ...
}

// ------ C.Nested.cs
partial class C
{
    public class Nested
    {
        // ...
    }
}

// ------ C.cs

public partial class C : IFoo
{
    // ...
}

// ------ C.Nested.cs
partial class C
{
    public class Nested
    {
        // ...
    }
}