# Non-Manifold Topology

Each vertex in polygonal mesh belongs to none (isolated vertex) or several polygons. All the polygons that are adjacent to a given vertex and share a common edge form a fan. The topology is manifold if for each vertex there is exactly one fan.

Tip

Having a **non-manifold** topology in production basemesh is often not a good practice unless you know what you are doing.
Many times **non-manifold** meshes are created by mistake and can later cause problems with editing, rendering etc.

Below are some examples of non-manifolds:

## Non-manifold cases include:

- singular vertex:
more than two fans are adjacent to the given vertex

- non-manifold edge:
more than two polygons are adjacent to single edge

- unoriented polygons:
two polygons that share a common edge have opposite orientation of normals

- isolated vertex:
vertex that does not belong to any polygon

- invalid polygon:
polygon that includes a given vertex multiple times

Some algorithms like wrapping, subdivision or decimation can not work with non-manifold topology. Please use RepairGeom node to convert it to manifold.

Note

Some nodes in Wrap like DeletePolygons can produce non-manifold topology.