> ## Documentation Index
> Fetch the complete documentation index at: https://docs.ntop.com/llms.txt
> Use this file to discover all available pages before exploring further.
# How to sweep along two rails?
## Question:
How does the **Sweep along Two Rails** block work?
## Answer:
The **Sweep along Two Rails** block creates an implicit body by sweeping a section along a path defined by two rail curves and a spine. Because the section's position, rotation, and scale are all driven by the rails, the resulting shape can taper, twist, and change size along its length, rather than staying constant like a basic sweep.
### Definitions
* **Rail:** The primary curves defining the trajectory of the swept section. They control the position, scale, and rotation of the section within the spine plane.
* *Valid:* Linear, nonlinear, nonplanar, closed
* *Invalid:* Non-monotonic with respect to the spine or another rail
* **Spine:** Controls the orientation of the *plane* of the moving section.
* *Valid:* Linear, nonlinear, nonplanar, closed
* *Invalid:* Non-monotonic with respect to a rail
* **Section:** The (implicit) planar shape at any given point along the sweep.
### Basic Controls
* **Trajectory:** The path along which the section is swept, defined by the two guide rails.
* **Rotation:** The in-plane rotation of the section being swept, controlled by the rails.
* **Orientation:** The orientation of the plane of the section at any point along the path. The orientation is always perpendicular to the spine.
* **Scale:** The in-plane scale of the section at any point along the path, controlled by the second rail (or a law). Scaling is typically uniform, but if a third rail is provided, non-uniform scaling is applied.
### Overloads and Trimming
The block can sweep either a **Profile** or a planar section of an **Implicit Body**. A shape to sweep and a starting plane are both required for the sweep — the Profile overload fully defines these using the input Profile's own properties.
*Note: The swept result is the exact planar slice of the body at the starting plane, not a profile created using Section Body.*
The **Implicit Body** overload instead requires both the body and a plane that defines the planar section of the body to sweep.
* **Caps:** When checked, closes the swept body with planar caps at the start and end of the spine.
## Example: Sweeping an airfoil to create a wing
To create a wing, the setup is as follows:
* **Profile:** The airfoil to sweep.
* **Spine:** A line in the spanwise direction. The airfoil sections remain orthogonal to this line, keeping them streamline-aligned.
* **Rail 1:** Leading edge.
* **Rail 2:** Trailing edge.
* **Caps:** True, to create a trimmed wing.
The block outputs an **Implicit Body** representing the swept geometry.
*Tip: If the swept result looks twisted or self-intersects, check that the Spine and Rail curves all start and end near the same relative locations, and that they run in a consistent direction. Rails that diverge significantly in length or shape from the spine can produce unexpected results.*
## Download:
[Sweep along Two Rails Example](https://storage.googleapis.com/files-learn/help-articles/implicit-modeling/sweep-along-two-rails.ntop)
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