At Underleau, we believe in a simple, unyielding truth: your words belong to you. In a world dominated by cloud-tethered applications that trap your thoughts on remote servers, we chose a different path: Local-First.

To deliver this vision without sacrificing the high-fidelity typography, pagination, and visual mapping that professional writers require, we built two custom local-first file formats from the ground up: .udl (Underleau Document Language) and .cudl (Canvas Underleau Document Language).

These are not just file extensions. They are a revolutionary step forward in document ownership.

Built for Local-First from the Ground Up

Most modern editors are built on database schemas designed for real-time collaboration first and local storage second. When you lose internet connection, they break or lock you out.

Underleau is the reverse.

Our formats are serialized as clean, standard UTF-8 JSON files that sit directly on your hard drive in a directory you control (your Vault). When you type, Underleau updates the file locally with zero latency and zero dependency on a network. The cloud is merely an optional helper for secure backup (Pro Sync) — the local files are always the canonical source of truth.

`.udl` — High-Fidelity Text with Block-in-Block Nesting

The .udl format is engineered for manuscripts, essays, and structured long-form writing.

Standard Markdown (.md) is wonderful for plain text but fails when you need high-fidelity layouts, precise font family overrides, highlighted text colors, or complex page pagination.

To solve this, .udl utilizes a custom hierarchical AST (Abstract Syntax Tree). It was designed specifically for block-in-block nesting:

A paragraph can contain formatted text with custom styles.
Lists can nest list items and check-lists recursively.
Tables can hold complex structured grids.
Headers, page breaks, and formatting options all nest cleanly without breaking the document flow.

This gives you the styling capabilities of a heavy desktop publishing suite like MS Word or Google Docs, but structured in a clean, logical tree format.

`.cudl` — Your Infinite Vector Sketchpad

Writers don’t just write; they think, brainstorm, map, and organize. That is where .cudl comes in.

The .cudl format is the container for Underleau's high-performance infinite canvas. It is optimized to hold:

Infinite coordinates of freehand vector drawings and annotations.
Stylized shapes (rectangles, diamonds, ellipses) and arrows connecting ideas.
Rich text cards and color-coded sticky notes.

Non-Interchangeable Paradigms

It is important to note that .udl and .cudl are fundamentally different and not interchangeable.

A .udl file is structured linear text that respects page breaks, line heights, and paragraph flow.
A .cudl file is a two-dimensional, zoomable infinite viewport that coordinates graphics and spatial relations.

They represent two different modes of creative thinking, coexisting side-by-side in your vault.

Zero Proprietary Lock-In

We hate proprietary locks. Your creative legacy should never be trapped in format jail.

Even though .udl and .cudl are namespaced to Underleau, they are fully open and human-readable under the hood. More importantly, Underleau features built-in, high-fidelity export workers. At any point, you can export your documents with a single click to:

Markdown (.md) for cross-compatibility with Obsidian or VS Code.
PDF for immediate distribution and printing.
Word (.docx) and OpenDocument Text (.odt) for sharing with traditional editors.
Plain Text (.txt) for raw copying.

You start with .udl/.cudl, but where you go is completely up to you.

Mind-Blowing File Size Efficiency

Traditional office formats are incredibly bloated. A blank Microsoft Word .docx file is actually a zipped archive filled with hundreds of XML files, default schema references, and heavy metadata, often weighing in at 20 KB to 50 KB before you even type a single word.

An average .udl file contains only the optimized JSON tree of your text.

A 5,000-word chapter in .udl takes up only about 15 KB of space.
It is 3x to 5x lighter than the equivalent Word document.

This extreme efficiency means that backups are instantaneous, cloud syncing uses minimal bandwidth, and you can store thousands of complete novels on a simple thumb drive.

Conclusion

The Underleau Document Language formats represent a return to software that respects the creator. Fast, lightweight, local-first, and completely open. Your words are too important to be locked in someone else's database.

Ready to experience local-first writing? Download Underleau today and take back ownership of your work.