# Join Overview

Joins are used to combine elements from different collections (e.g.
`ImageCollection` or `FeatureCollection`) based on a condition
specified by an `ee.Filter`. The filter is constructed with arguments for
the properties in each collection that are related to each other. Specifically,
`leftField` specifies the property in the primary collection that is related
to the `rightField` in the secondary collection. The type of filter (e.g.
`equals`, `greaterThanOrEquals`, `lessThan`, etc.)
indicates the relationship between the fields. The type of join indicates one-to-many
or one-to-one relationships between the elements in the collections and how many matches
to retain. The output of a join is produced by `join.apply()` and will vary
according to the type of join.