Mirror symmetries naturally produce patterns without visible divisions
between repeated units. For other symmetries, you can create
seamless patterns by blending (melding together) neighboring units
horizontally and vertically using the Blend H and Blend V
sliders in the Parameters panel.
For symmetries that allow different amounts of blending in the horizontal and
vertical directions, you can change the horizontal and vertical blending
independently. Otherwise, you can change only the horizontal blending and
SymmetryMill either adjusts the vertical blending accordingly, as dictated
by symmetry laws, or turns the vertical blending off when it is not
necessary because the pattern is already seamless in the second direction.
Note: Both horizontal and vertical blending are disabled for
symmetries that involve two mirrors and therefore achieve seamless patterns