# Constraints Constraints define how the motion of certain degrees of freedom (DOFs) at one node depends on the motion of DOFs at another node. In other words, constraints link nodes together so that their movements follow specified relationships. They are often used to: Tie two nodes to move together (rigid links) Enforce symmetry or anti-symmetry conditions Define proportional motion between nodes (e.g., gear or lever effects) MPCs reduce the model’s independent DOFs and must be used carefully to avoid over-constraining the system. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://alpaca4d.gitbook.io/docs/basics/constraints.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.