More than one datum used to establish the relationship of toleranced features.

• Global -  thought of as a datum system for an assembly
  
• Local - thought of as a datum system for individual products that are part of the overall assembly where the local datum system is offset from the global datum system.
  

The global datum system of an aircraft may be defined at a specified location for all the assembly parts to be referenced back to. However, all the individual components may have local datum systems.

The global datum is relevant to the whole assembly and the local datum is relevant to individual components of the assembly.

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Good practice online modules

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• Dimensional
• Overview of basic design and interpretation of an engineering drawing
• An introduction to datum
• An introduction to geometric tolerancing
• An introduction to the designer's role when designing a component
• An introduction to the manufacturer's role when designing a component
• An introduction to the inspector's role when designing a component