# Why do model and source YAML files always start with \`version: 2\`?

Once upon a time, the structure of these `.yml` files was very different (s/o to anyone who was using dbt back then!). Adding `version: 2` allowed us to make this structure more extensible.

From [dbt Core v1.5](<https://docs.getdbt.com/docs/dbt-versions/core-upgrade/Older versions/upgrading-to-v1.5.md#quick-hits>), the top-level `version:` key is optional in all resource YAML files. If present, only `version: 2` is supported.

Also starting in v1.5, both the [`config-version: 2`](../../reference/project-configs/config-version.md) and the top-level `version:` key in the `dbt_project.yml` are optional.

Resource YAML files do not currently require this config. We only support `version: 2` if it's specified. Although we do not expect to update YAML files to `version: 3` soon, having this config will make it easier for us to introduce new structures in the future

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