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# 5 Geometry module

Gmsh’s geometry module provides a simple CAD engine, using a boundary representation (“BRep”) approach: you need to first define points (using the `Point` command: see below), then lines (using `Line`, `Circle`, `Spline`, …, commands or by extruding points), then surfaces (using for example the ```Plane Surface``` or `Ruled Surface` commands, or by extruding lines), and finally volumes (using the `Volume` command or by extruding surfaces).

These geometrical entities are called “elementary” in Gmsh’s jargon, and are assigned identification numbers (stricly positive) when they are created:

1. each elementary point must possess a unique identification number;
2. each elementary line must possess a unique identification number;
3. each elementary surface must possess a unique identification number;
4. each elementary volume must possess a unique identification number.

Elementary geometrical entities can then be manipulated in various ways, for example using the `Translate`, `Rotate`, `Scale` or `Symmetry` commands. They can be deleted with the `Delete` command, provided that no higher-dimension entity references them. Zero or negative identification numbers are reserved by the system for special uses: do not use them in your scripts.

Groups of elementary geometrical entities can also be defined and are called “physical” entities. These physical entities cannot be modified by geometry commands: their only purpose is to assemble elementary entities into larger groups, possibly modifying their orientation, so that they can be referred to by the mesh module as single entities. As is the case with elementary entities, each physical point, physical line, physical surface or physical volume must be assigned a unique identification number. See Mesh module, for more information about how physical entities affect the way meshes are saved.

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