# Concepts

The quick start shows how to use plesty-lib. This chapter explains how it works
underneath: how data moves during a measurement, how an instrument is checked before
use, how the library is divided, and why it is built this way. You do not need it to
write a module, but it answers the questions that come up when something behaves
unexpectedly, or when you want to change plesty-lib itself.

| Page | Explains |
|---|---|
| [Where a Measurement's Data Goes](data_flow.md) | the run folder, how large files reach it, and how a running measurement is paused or stopped |
| [Preflight: is the instrument actually there?](preflight.md) | the check before a measurement, from the connection up to the experiment |
| [Packages and Layers](packages.md) | the parts of plesty-lib and which may use which |
| [Interface Contracts](contracts.md) | what each part promises to the code built on it |
| [Design Decisions](design_decisions.md) | why plesty-lib is built this way, and what each choice costs |
| [Dependencies](dependencies.md) | the packages plesty-lib requires, and the rules for adding one |