# Relative Boundary conditions

**URL:** <https://community.freefem.org/t/relative-boundary-conditions/1616>\
**Category:** Feature Request\
**Created:** [March 21, 2022, 8:33pm UTC](https://community.freefem.org/t/relative-boundary-conditions/1616 "2022-03-21T20:33:10Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![mahdi](https://avatars.discourse-cdn.com/v4/letter/m/e9c0ed/32.png) [@mahdi](https://community.freefem.org/u/mahdi)\
**Post date:** [March 21, 2022, 8:33pm UTC](https://community.freefem.org/t/relative-boundary-conditions/1616/1 "2022-03-21T20:33:10Z")

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The relative Boundary conditions is a boundary condition where a relation exists between two Boundaries. (The simplest type is a periodic one which equate two Boundary conditions.)  
For instance P|B=P|A +const.  
This can be an important update especially for fluid flow and pressure driven problems.

 ![Untitled2](https://canada1.discourse-cdn.com/flex030/uploads/freefem/original/2X/6/60c577de377edb46c9691f6f048a6832ab0c3716.png)

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**Author:** ![frederichecht](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/frederichecht/32/15_2.png) [@frederichecht](https://community.freefem.org/u/frederichecht)\
**Post date:** [March 22, 2022, 4:03pm UTC](https://community.freefem.org/t/relative-boundary-conditions/1616/2 "2022-03-22T16:03:38Z")

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For periodic boundary condition see the documentation or exemple.