# Mapping

As you build a transformation, you may notice a sequence of steps that you want to repeat. This repetitive part can be turned into a mapping.

A mapping is a transformation with placeholder input and output steps. The mapping transformation is executed through the Mapping step in a parent transformation. Because the parent transformation runs a separate transformation through a specific step, the mapping transformation is commonly referred as a sub-transformation. The sub-transformation must contain the following input and output steps:

* Mapping input: a placeholder where the mapping expects input from the parent transformation.
* Mapping output: a placeholder indicating, from where, the parent transformation can read data.

Use mapping when you want to re-use a certain sequence of steps in a transformation. The following image illustrates mapping and the relationship of the parent

![Transformation Mapping](/files/iSkzrm4sB92nTWFktJ96)


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