> For the complete documentation index, see [llms.txt](https://alpaca4d.gitbook.io/docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://alpaca4d.gitbook.io/docs/basics/constraints.md).

# Constraints

A **constraint** makes the motion of one node depend on the motion of another. Where a [Support](/docs/basics/elements/support.md) holds a node still, a constraint ties nodes to each other.

Constraints remove independent degrees of freedom from the model, which is what makes them cheap — and what makes them easy to over-use. Two constraints that close a loop, or that both claim the same DOF, will make the system singular.

## Components in this tab

| Component                                                | Nickname          | Ties                                                                                |
| -------------------------------------------------------- | ----------------- | ----------------------------------------------------------------------------------- |
| [Rigid Diaphragm](/docs/basics/constraints/diaphragm.md) | `Rigid Diaphragm` | A group of nodes into a plate that is rigid in its own plane.                       |
| [Rigid Link](/docs/basics/constraints/rigid-link.md)     | `Rigid Link`      | One node to another, as a rigid body.                                               |
| [Equal DOF](/docs/basics/constraints/equal-dof.md)       | `Equal DOF`       | Chosen degrees of freedom of one node to another, ignoring the offset between them. |

## Choosing between them

| You want                                                                                      | Use                                                           |
| --------------------------------------------------------------------------------------------- | ------------------------------------------------------------- |
| A floor slab that is stiff in-plane                                                           | **Rigid Diaphragm**, one per storey                           |
| Two offset nodes to move together as a rigid body                                             | **Rigid Link**, `beam`                                        |
| Some degrees of freedom to read the same at two nodes — a shear key, a slider, a symmetry tie | **Equal DOF**                                                 |
| A join with real flexibility                                                                  | a [Spring Link](/docs/basics/elements/spring-link.md) instead |
| A node held still                                                                             | a [Support](/docs/basics/elements/support.md) instead         |

{% hint style="info" %}
Constraints are solved by the **Constraint handler** set on [Analysis Settings](/docs/basics/analysis/analysis-settings.md). The default, `Transformation`, handles rigid diaphragms and rigid links correctly; `Plain` does not and will quietly give the wrong answer. Only change it if you know why.
{% endhint %}