To apply the coincident mate to our assembly, follow these steps:

1. To apply mates, select the Mate command, which can be found under the Assembly command, as shown in the following screenshot:

2. After selecting Mate, we will see the following window. The layout is very similar to all the other features we've used with PropertyManager—it's on the left-hand side of the screen. At this point, we will be asked to select which elements we want to mate. This can include surfaces, edges, and points. For this selection, we'll select the top surfaces of the base and the triangle, as shown in the following image. These will fill in the Mate Selections space, which is highlighted in red in the following image:

After the selection, we will see that the two parts will move in relation to each other. Also, one mate will be selected automatically. In this case, the mate will be Coincident. We can change this mate if we want to use another one. However, in this case, Coincident will work for us. Our canvas will look as follows. To apply the mate, click on the green checkmark on top of the mate's PropertyManager:

When we try moving the triangle around the canvas, we will notice that it can only move sideways to the base. However, it won't move up and down relative to the base. Now, let's apply another coincident mate to restrain the triangle more. Note that the mate command is still active, which means we can apply more mates.

3. Select the edges shown in the following image. These are the outer edges of the triangle and the inner edge of the base. Again, SOLIDWORKS will interpret that we want the coincident mate and automatically apply it:

After selecting these two edges, we will see the following image. Before applying the mate, please take note of the following: 

• In the canvas, the two parts will move to preview the mate. Make sure that the preview matches our needs. 
• The Mate Selections list in the PropertyManager. Here, we can double-check that we have selected two edges. If not, we can delete the undesired selections there. 
• The Standard Mate selection in the PropertyManager. Here, we can double-check that the selected mate is what we want to apply. If we want to use another mate, we can select it from there.

4. After conducting all the checks, we can click on the green checkmark to apply the mates:

This concludes our application of the coincident mate. Next, we will apply the perpendicular mate to our assembly.