Symmetry Modifier

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Modify panel > Make a selection. > Modifier List > Symmetry

Make a selection. > Modifiers menu > Mesh Editing > Symmetry

The Symmetry modifier is unique in that it allows you to perform three common modeling tasks:

Mirror a mesh about an X, Y, or Z plane.
Slice a mesh, removing parts if necessary.
Automatically weld vertices along a common seam.

Examples of using Symmetry with different mirror axes or by moving the mirror gizmo

You can apply the Symmetry modifier to any geometry, and you can animate the mirror or slicing effect by animating the modifier's gizmo.

When the Symmetry modifier is applied to a mesh, any edits made to the original half of the mesh will appear interactively to the other half.

The Symmetry modifier is especially useful when modeling characters or building ships and aircraft.

Note: The Symmetry modifier converts Editable Mesh, Editable Patch, and NURBS objects to Editable Meshes in the modifier stack; however, Editable Poly objects remain as polys.

Procedure

Example: To apply the symmetry modifier to an object:

Create a teapot in the perspective viewport.
Apply the Symmetry modifier.

The teapot will look like it has two spouts.
In the modifier stack, click the + button to see the Mirror gizmo, and then select Mirror.

The mirror gizmo acts as a slice plane when it is within the boundaries of the object
With the Mirror Axis set to X, click and drag the mirror gizmo along the X axis.

Dragging right slices more of the teapot until there is nothing visible. Dragging left causes a second teapot to appear. When the mirror gizmo is moved beyond the boundaries of the original mesh, it acts as a mirror plane showing you two complete teapots.

Interface

Modifier Stack

Mirror—The placement of the mirror gizmo delegates how the object will be affected by symmetry. You can move or rotate, as well as animate the gizmo.

For more information on the stack display, see Modifier Stack.

Parameters rollout

Mirror Axis group

X, Y, Z—Specify the axis about which the symmetry takes place. You can see the effect in the viewport as you select the axis.

Flip—Turn on Flip if you want to flip the direction of the symmetry effect. Default=off.

Slice Along Mirror—Turning on Slice Along Mirror causes the mirror gizmo to act as a slice plane when it in located inside the boundaries of a mesh. When the gizmo is outside the boundaries of a mesh, the symmetrical reflection is still treated as part of the originating mesh. If Slice Along Mirror is turned off, the symmetrical reflection is treated as a separate element of the originating mesh. Default=on.

Weld Seam—Turning on Weld Seam assures that the vertices along the mirror axis will be automatically welded if they are within the Threshold. Default=on.

Threshold—The value of the Threshold setting delegates how close vertices can be before being automatically welded together. Default=0.1.

Note: Setting the Threshold value too high may result in some distortion of the mesh, especially when the mirror gizmo is outside the boundaries of the originating mesh.