# Parallel ruler with crossed arms and slots

> How does a parallel ruler stay parallel?

- Category: Mechanisms › Linkages
- Kind: Hardware
- Basis: Xem Studio design following Brown No. 324 (1868)
- Page: https://xemstudio.com/objects/parallel-ruler-crossed-arms-mechanism/ · JSON: https://xemstudio.com/api/geometry?object=parallel-ruler-crossed-arms-mechanism · Plain text: https://xemstudio.com/objects/parallel-ruler-crossed-arms-mechanism/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses
- Use in every chat: add https://xemstudio.com/mcp as a connector (https://xemstudio.com/use-with-ai/)

**Exact dimensions.** True size, with no printing tolerance added.

## Notes

- Two straight-edges joined by two equal crossed arms pinned at their middles; each arm is pinned to one blade and slides in a slot of the other. The pinned ends are always one above the other, so the blades stay parallel and square to each other at any opening.
- Built from two 200 mm blades and two 80 mm centre-hole links: drawn open 40 mm; the slot lets it open from 0 to 55 mm.
- Brown's No. 324 (compound parallel ruler, crossed arms with slot-and-pin).
- Parts (each has its own page): parallel-ruler-blade-200, centre-hole-link-80, dowel-pins-metric.
- Brown's movement No. 324 (1868) shows it; this model is a Xem Studio design following his figure, every part at true size.

## Sizes

| parameter | value |
|---|---|
| blades | 2 × 200 × 20 × 5 mm |
| arms | 2 × 80 mm, crossed at the middle |
| opening | 0–55 mm (drawn 40) |

## Where the files are

- Henry T. Brown (1868, public domain): [Five Hundred and Seven Mechanical Movements — scan on archive.org](https://archive.org/details/fivehundredseven00browiala) — web — Publisher's page; dimensions are facts
- 507movements.com: [Brown's movement No. 324: figure, his text and an animation](https://507movements.com/mm_324.html) — web — Publisher's page; dimensions are facts

## Free STEP/STL (1 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/parallel-ruler-crossed-arms-mechanism/index.json

- parallel-ruler-crossed-arms-mechanism-assembly: [STEP](https://xemstudio.com/files/hardware/parallel-ruler-crossed-arms-mechanism/parallel-ruler-crossed-arms-mechanism-assembly.step) · [STL](https://xemstudio.com/files/hardware/parallel-ruler-crossed-arms-mechanism/parallel-ruler-crossed-arms-mechanism-assembly.stl) — Assembled mechanism, all parts placed at one pose (STEP keeps each part separate)

## Questions

**How does a parallel ruler stay parallel?**

Two straight-edges joined by two equal crossed arms pinned at their middles; each arm is pinned to one blade and slides in a slot of the other. The pinned ends are always one above the other, so the blades stay parallel and square to each other at any opening.

## Fits with

- [Parallel ruler blade 200 × 20 × 5, pivot hole and 3 mm slot](https://xemstudio.com/objects/parallel-ruler-blade-200/)
- [Centre-hole link, 80 mm (three Ø3 holes: −40, 0, +40)](https://xemstudio.com/objects/centre-hole-link-80/)
- [Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)](https://xemstudio.com/objects/dowel-pins-metric/)