From BlenderWiki

Vertex Groups

A mesh is a set of connected Vertices, sometimes thousands of vertices for the more complex objects. Blender allows you to group these vertices for several main reasons:

• Re-using parts of a mesh for making copies
• Hiding “everything else” while you work on details
• Documentation and explanation to others
• Armatures deformation
• Generating particles from only the group
• Controlling the velocity of particles emitted
• Assigning multiple materials to a single mesh

Why use Vertex Groups?

Vertex groups identify sub-components of an object, like the legs of a chair or the hinges of a door. By “bookmarking” such regions in vertex groups you can easily select and work on them in isolation without having to create separate objects. With the hide function you can even remove everything else from view.

Vertex groups also make it easy to cull out and duplicate a part of the mesh many times. Consider modeling a Lego block. The most simple block consists of a base and a nipple. To create a four-nipple block, you would want to be able to easily select the nipple vertices, and, still in Edit mode, duplicate them and position them where you want them.

Another use for vertex groups is for skinning an armature. If you want to animate your mesh and make it move, you will define an armature which consists of a bunch of invisible bones. As a bone moves, it deforms or moves the vertices associated with it. Not all of the vertices, but some of them; the ones assigned to it. So, when you move the bone “Arm”, the arm bone moves the “Arm” vertices, and not the “Leg” vertices. In this way, parts of the mesh can stretch and move around, while other parts remain stationary.

The Particle System menu has a Vertex group panel where various properties can be limited to vertex groups. The vertex group's weight painting can control the amount, size, and velocity of the particles. For example, Hair — a kind of particle system — can use a vertex group named Scalp to have the hair emitted from only a part of the skull.

Additionally, many Modifiers use Vertex Groups to limit their influence. Some modifiers, such as the Mask Modifier, need a vertex group to have any effect at all.

Creating and Deleting

Vertex Groups panel in edit mode

By default, an object does not have any groups, and all of its vertices are hanging out there in cyberspace as loners. While vertex groups can be automatically generated in some contexts, for example when weight painting, we typically create and modify vertex groups in the Vertex group panel in the Object data menu. Once a vertex group has been added, and when we are in Edit Mode, the vertex group panel expands to display a row of buttons.

Vertex Groups popup menu

To create a vertex group, LMB click the + button. When you do, a new vertex group (named, surprisingly, “Group”) is created, and the panel shows you a Weight numeric slider/entry/scroll box. Any selected vertices are not yet assigned to the new vertex group, you must click the Assign button to actually allocate vertices to the newly created vertex group. Note that using the Shortcut CtrlG » Add to New Group, you do all this in one step.

To delete a vertex group, select it from the list and click the - button. Yes, it’s as simple as that. Any vertices that belonged to that group are unassigned from that group. However, please keep in mind that vertices can belong to many groups. When they are unassigned from one group, they still belong to their other groups.

To name a group something other than the creative “Group”, ⇧ ShiftLMB click the name field, and simply type in the name you want. Choosing short, self-explanatory names is crucial in larger projects involving many users.

Selecting and Deselecting

From experience, we have found that it is best to start first by seeing the existing vertices in a group, before adding more or removing some. To do this, first de-select all vertices by pressing A once or twice in the 3D view. Then, with the appropriate group active, press the Select button. In your 3D window, the vertices that belong to the active group will be selected and highlighted.

Sometimes you will want to see if any vertices are still loners. To do so, select All the vertices in the 3D window. For each Vertex Group, LMB click the Deselect button to de-select the vertices in that group. Repeat the de-selection for each group. When you run out of groups, any vertices left highlighted are the loners. Sort of like picking baseball teams.

Assigning and Removing Vertices

To add vertices to a group you do the following:

1. Select the group you want to work with from the group list.
2. Use your mouse to ⇧ ShiftRMB select more vertices that you want in that group.
3. LMB click the Assign button, or make CtrlG » Add Selected to Active Group.

Keep in mind that a vertex can be assigned to multiple groups.

To remove vertices from a group:

1. Select the group you want to work with from the group list.
2. Select the vertices you want to remove from the vertex group.
3. LMB click the Remove button, or use CtrlG » Remove from Active Group.

Vertex Group Management

Vertex groups panel's dropdown menu

Sort Vertex Groups

Sort vertex groups alphabetically

Copy Vertex Group

Create a copy of the active vertex group (i.e. vertices assigned to the active group are automatically assigned to the new group.)

Copy Vertex Group to Linked

Copy vertex groups to all linked duplicates

Copy Vertex Group to Selected

Copy vertex groups to other selected meshes

Mirror Vertex Group

Mirror all vertex groups, flip weights and/or names, editing only selected vertices, flipping when both sides are selected otherwise copy from unselected

Delete All

Delete all vertex groups