What Is YAML? A Plain-English Guide (2026)

What does YAML stand for?

YAML stands for YAML Ain't Markup Language. It's a recursive acronym - the YAML at the start of the expansion is the same YAML being defined. The format was originally proposed in 2001 with the expansion Yet Another Markup Language, then renamed a year later to clarify that YAML is for data, not markup. The current spec is YAML 1.2, published in 2009 and revised in 2021.

The full form is sometimes spelled out as YAML Ain't a Markup Language. Both spellings appear in the official documentation and both refer to the same format.

The .yaml file format

A YAML file is plain UTF-8 text with the .yaml or .yml extension. Both extensions are identical - .yaml is the official recommendation, but .yml is widely used because some legacy tools assume three-character extensions.

YAML is whitespace-sensitive. Indentation - always with spaces, never tabs - defines the structure of nested maps and lists. The file format defines:

• Maps - key-value pairs separated by a colon and a space.
• Lists - items prefixed by a hyphen and a space.
• Scalars - strings, numbers, booleans, and null.
• Comments - any text after a hash character on a line.
• Anchors / aliases - reusable references with &name and *name.
• Block scalars - multiline strings using | for literal text or > for folded lines.

YAML syntax in 60 seconds

Here are the elements you'll use in 95% of YAML files:

A full YAML example

Here is a complete YAML file showing maps, lists, comments, booleans, and a multiline string:

# Sample YAML configuration
name: my-app
version: 1.4.2
production: true
servers:
  - host: api.example.com
    port: 443
  - host: web.example.com
    port: 80
features:
  auth: enabled
  rateLimit: 100
description: |
  A multiline description
  spanning two lines.

Try this in our YAML editor to see live validation and syntax highlighting, or paste it into the YAML to JSON converter to see the equivalent JSON.

YAML vs JSON

YAML and JSON describe the same data shapes, so any YAML document can be converted to JSON. The differences are about ergonomics and feature set:

Same data, two formats:

# YAML
name: my-app
version: 1.4.2
production: true

# Equivalent JSON
{
  "name": "my-app",
  "version": "1.4.2",
  "production": true
}

For a deeper comparison, read our dedicated YAML vs JSON guide.

Where YAML is used

YAML is the default config format across modern infrastructure and developer tooling:

• Kubernetes - manifests for deployments, services, ingress, and every other resource use YAML.
• GitHub Actions - workflow files in .github/workflows/.
• Docker Compose - docker-compose.yaml service definitions.
• Ansible - playbooks, inventory, and role definitions.
• OpenAPI / Swagger - API specifications.
• GitLab CI / CircleCI / Travis - pipeline config.
• Helm - chart values and templates.
• Hugo / Jekyll - site config and post front matter.
• Cloud-init - server bootstrap config for AWS, GCP, and Azure.

Free YAML tools on this site

Our YAML toolset covers the most common day-to-day tasks:

• YAML Validator - live syntax checking with line-and-column errors.
• YAML to JSON Converter - convert YAML to JSON with anchor expansion.
• JSON to YAML Converter - convert JSON to YAML with custom indent and key sorting.
• YAML Formatter - beautify and normalize YAML for diff-clean commits.
• YAML Editor - browser editor with live validation and autosave.

For specific topics, see our guides on YAML comments, multiline strings, and .yml vs .yaml.