LilyPond Contributor's Guide: 10.9.2 Write regression tests

### 10.9.2 Write regression tests

In order to demonstrate that the code works properly, you will need to write one or more regression tests. These tests are typically ‘`.ly`’ files that are found in ‘`input/regression`’.

Regression tests should be as brief as possible to demonstrate the functionality of the code.

Regression tests should generally cover one issue per test. Several short, single-issue regression tests are preferred to a single, long, multiple-issue regression test.

If the change in the output is small or easy to overlook, use bigger staff size – 40 or more (up to 100 in extreme cases). Size 30 means "pay extra attention to details in general".

Use existing regression tests as templates to demonstrate the type of header information that should be included in a regression test.

#### LilyPond — Contributor’s Guide v2.24.4 (stable-branch).