> ## Documentation Index
> Fetch the complete documentation index at: https://docs.spine.altician.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Midline Tool

> Define the body's center reference plane for symmetric measurements

## What Is the Midline?

The midline is a reference plane representing the body's central axis — the line that divides the body into left and right halves. Once defined, it serves as a fixed reference for measurements and visual orientation.

## Defining the Midline

The midline is defined by drawing reference lines in **two separate viewports** (for example, one in the axial view and one in the coronal view). This two-point definition gives the plane full 3D orientation.

1. Select the **Midline** tool from the toolbar.
2. In the first viewport, click to draw a line along the center of the anatomy.
3. Switch to a second viewport and draw a second reference line.
4. The midline plane is calculated and displayed automatically.

<Note>
  For best accuracy, use bony landmarks (such as the spinous processes) as guides when drawing midline lines.
</Note>

## Midline Display

Once defined, the midline appears as a translucent plane overlaid on the 3D volume. It is visible in both the **main viewport** and the **Scouter** viewport.

## Toggle Midline Visibility

Click the eye icon next to **Midline** in the right panel to show or hide the plane. Hiding the midline does not delete it — it remains defined and can be shown again at any time.

## Midline as a Measurement Reference

The midline is used automatically in the **S-M Distance** measurement, which calculates the surface path from a skin entry point to the midline. See [S-M Distance](/tools/sm-distance) for details.

<Warning>
  The midline is a user-defined reference plane. It is not detected automatically from the anatomy — take care to draw the lines accurately along the intended central axis.
</Warning>