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Paraphrasing the F# language guide's [`let` Bindings in Classes][1] article:

> A **`let` binding** in a class creates a *private* field or function;
> to expose data or functions publicly, declare a property or a member
> method.
> 
> + An **instance `let` binding** is a `let` binding that is not preceded by the `static` keyword. 
> 
>   > Instance `let` bindings execute when objects are created. 
> 
> + **Static `let` bindings** are part of the static initializer for the class.
> 
>   > Static `let` bindings are guaranteed to execute before the type is first used.

If one tried to refactor the `Vector2D` type by removing the `static` keywords before `zero`, `onex`, and `oney` private functions,

```fsharp
type Vector2D(dx : float, dy : float) =
     let zero = Vector2D(0.0, 0.0)
     let onex = Vector2D(1.0, 0.0)
     let oney = Vector2D(0.0, 1.0)
     /// Get the zero vector
     static member Zero = zero
     /// Get a constant vector along the X axis of length one
     static member OneX = onex
     /// Get a constant vector along the Y axis of length one
     static member OneY = oney
```

then they would have received

```
error FS0039: The value or constructor 'zero' is not defined.
```

as `zero` and co. are only guaranteed to evaluate when the class is instantiated.


  [1]: https://learn.microsoft.com/en-us/dotnet/fsharp/language-reference/members/let-bindings-in-classes