---
title: 'Thread, needle, fabric'
part: 'Part 0 — Start Here'
partNumber: 0
order: 4
subtitle: 'A five-minute physics primer'
---

A machine embroidery needle doesn't draw. It **punches penetrations** through fabric and pulls thread taut between them. What you see in a preview is the thread pulled between those points — a series of tiny line segments, not a continuous stroke.

Understanding this changes how you read designs and how you debug them.

## What the machine actually does

Each stitch is two holes: the needle goes down, locks with the bobbin thread, comes back up. The preview shows the path between consecutive holes. The dashes you see in jumps are _thread floating over the fabric_ — not sewn, just carried.

Three things determine what you see on the fabric:

1. **Where thread goes** — the path of the turtle
2. **How densely it piles** — stitch length, fill spacing, number of layers
3. **How it's secured** — lock stitches at start/end, trims, colour stops

## Direction is visible

Embroidery thread has gloss. Stitches angled toward a light source reflect more than those angled away. This means **stitch direction is a colour channel** — two adjacent fills at 0° and 90° look different even with the same thread.

The book will return to this many times. For now, try this:

<Run>
{`// Two adjacent squares — same thread, different angle
fillangle 0
beginfill
  setxy -12 -10
  setxy  -2 -10
  setxy  -2  10
  setxy -12  10
endfill

moveto -2 0

fillangle 90
beginfill
setxy 2 -10
setxy 12 -10
setxy 12 10
setxy 2 10
endfill`}

</Run>

The left square has horizontal rows, the right square has vertical rows — one thread colour, two apparent shades.

## The vocabulary you'll meet

| Term               | What it means                                                     |
| ------------------ | ----------------------------------------------------------------- |
| **Running stitch** | Thread follows the path directly, one line of stitches            |
| **Satin**          | Zigzag column — thread crosses a spine back and forth             |
| **Tatami / fill**  | Grid of rows densely covering a region                            |
| **Underlay**       | Lightweight stitches sewn _before_ the top layer to anchor fabric |
| **Trim**           | A machine cut between two areas of different thread               |
| **Lock**           | Tie-in / tie-off stitches that secure thread ends                 |

Each of these gets its own chapter. For now, just recognise the words.

<Checkpoint chapterId="ch-0-4">
  Run the two-squares example. Look closely at the preview — do you see a difference between the
  left and right squares? That's the gloss angle at work. Mark complete when you've observed it.
</Checkpoint>