As the name suggests, mirroring a sketch means to reflect one or more sketch entities around a mirroring line. It is very similar to reflecting an image in a mirror. The following diagram illustrates the components of mirroring in SOLIDWORKS:

In the preceding diagram, we can see that there are two parts that we need in order to use mirroring:

•  Sketch entities to mirror
•  A mirroring or reflection line

Since the two shapes are mirror sketch entitles of each other, any changes that happen to one shape will automatically happen to the other.

To highlight how we can use mirroring, we will create a shape. Note that the shape is a right-angle triangle with a mirrored reflection. We will use the MMGS measurement system for this exercise:

To create this shape, we need to complete the outlining and defining stages. Here, we will create the base sketch and then mirror an outline. Then, we will define the resulting sketch. 

Let's start by outlining our general shape and applying the sketch Mirror Entities command:

1. Sketch the outline of the first triangle and the mirroring line and use a centerline or construction line for the mirroring line:

2. Select the Mirror Entities command, as shown in the following screenshot:

3. On the left-hand side, we will see the available Mirror options. For Entities to mirror, select the three lines that make up the right-angle triangle (L2, L3, and L4). For Mirror about, select the centerline (L1). Make sure that the checkbox for Copy is checked. Now, we will see a preview of the triangular mirrored shape on the left. Click on the green checkmark to approve the mirroring:

To find out more about the mirroring effect, click and hold any of the blue parts of the sketch and move it. You will notice that the mirrored entity will reflect the same movement as the original shape.