Mirroring in Silo.
Oftentimes you'll be building a model (particularly characters) where you want the left side and the right side to be symmetrical.  Silo provides several options to make symmetrical modeling easy and intuitive, while keeping the overall program very simple.
In Silo, there are two different types of mirroring operations.  The first is the Mirror command in the Modify menu.  This command will mirror your selection across the axis specified in the Mirror Options dialog (accessed by clicking on the ... next to the Mirror command in the Modify menu).  This tye of mirror operation actually modifies the geometry of your shape.  Changes made to one half after the mirror operation will not be reflected on the other half.


This is the Mirror Options dialog.  In it you can choose the axis you want to mirror your selection over.  You can also choose the merge tolerance.  This determines how close vertices can be to the mirror seam before they get merged into the seam.


The second kind of mirror operation you can do in Silo is creating an Instance Mirror object.  An instance of an object is a non-editable copy of the object that automatically updates whenever you update the original object.  An Instance Mirror object is an instance which has been automatically scaled to be symmetrical to it's parent object across the axis specified in the Instance Mirror Options dialog.

The command to create an Instance Mirror object is in the Create menu (Create > Instance Mirror) and options are opened by clicking on the ... next to the Create Instance Mirror command.  A mirrored instance of the selected object will be created.
To work with instance mirroring you may want to start with a subdivided cube, deleting the half where the instance will go.


Then select this object and create an Instance Mirror object (here we have chosen from the options dialog to mirror along the x axis).


Edits made on the original object (on the right) will automatically show in the instance mirror object.


You may find that the seam between the two halves of this model "opens up" as you're performing your edits to the mesh.  You can prevent this from happening by enabling seam preservation.  This check item is found in Select > Enable Seam Preserve.  This option will ensure that any points that are on open edges (edges with one face attached) of an object with an instance mirror of itself will be pulled onto the seam plane if they are within the merge tolerance specified in the Mirror Options dialog.


You may wish to model with an Instance Mirror object, and when you're done modeling you can use the Mirror command to create real geometry on the other half of your model.