Not everything you'll want to select with the Polygonal Lasso Tool will be as simple as a four-sided billboard, but the steps are always the same. Simply click to add points along the object at the spots where your selection outline needs to change direction, then click back on the initial starting point to complete the selection.

Here's a photo of an old building. I want to replace the sky in the photo, which means I'll need to select the sky, drawing part of my selection around the top and sides of the building. Since the building is made up almost entirely of straight, flat surfaces, the Polygonal Lasso Tool should make it easy:

I'll begin my selection somewhere along the left side of the building by clicking to set my starting point, then I'll slowly make my way around the outside of the building, clicking to add points as needed. I'll zoom in a little to make it easier to see what I'm doing by pressing Ctrl++ (Win) / Command++ (Mac) a couple of times. To scroll the image around inside the document window, hold down your spacebar, which temporarily switches you to the Hand Tool, then click and drag the image to move it. Release your spacebar to switch back to the Polygonal Lasso Tool:

Switching Between The Polygonal Lasso Tool And The Standard Lasso Tool

As I make my way along the top of the building, I come across what appears to be a problem. Part of the design in the roof is actually rounded, which is bad news for the Polygonal Lasso Tool since it can only draw straight-sided selections. Fortunately, Photoshop makes it easy to switch between the Polygonal Lasso Tool and the standard Lasso Tool for occasions such as this. Simply hold down your Alt (Win) / Option (Mac) key, then begin dragging with your mouse. This temporarily switches you to the standard Lasso Tool, and with it, we can easily trace around any rounded or curved areas of an object:

Once you've traced along the edge of the rounded or curved surface, release your Alt / Option key, then release your mouse button. You'll switch back to the Polygonal Lasso Tool, at which point you can continue moving around the object and clicking to add more points:

Once I've finished drawing my selection around the building, I'll make sure I get all of the edge pixels in the sky along the sides and top of the photo by clicking with the Polygonal Lasso Tool into the gray pasteboard area around the photo. If you can't see the pasteboard area, press Ctrl+- (Win) / Command+- (Mac) a few times to zoom out until the pasteboard appears. Photoshop won't select the pasteboard, it will select only the pixels in the image:

To complete the selection, I'll click once again on my initial starting point, and with that, the sky in the photo is now selected:

I'm going to zoom back to the 100% zoom level by pressing Ctrl+Alt+0 (Win) / Command+Option+0 (Mac). If we look in my Layers panel, we can see that my document is made up of two layers. The photo of the building is on the top layer, and a photo of a dark, cloudy sky sits on the Background layer below it:

With the top layer selected, I'm going to hold down my Alt (Win) / Option (Mac) key and click on the Layer Mask icon at the bottom of the Layers panel. This converts my selection into a layer mask, and we can see that a layer mask thumbnail has been added to the top layer. Normally, the object or area that was selected would remain visible in the document while everything that was not selected would be hidden from view, but by holding down the Alt / Option key, I inverted the layer mask, which will hide the sky (the selected area) and keep the building (the unselected area) visible:

With the sky in the building photo now hidden, the clouds in the photo below it show through in the document:

Removing A Selection

In the example above, the selection outline disappeared when we converted it to a layer mask, but normally, when you're done with a selection created with the Polygonal Lasso Tool, you can remove it by going up to the Select menu at the top of the screen and choosing Deselect, or you can press the keyboard shortcut Ctrl+D (Win) / Command+D (Mac). You can also simply click anywhere inside of the document with the Polygonal Lasso Tool or with any of Photoshop's other selection tools.

Coming up, we'll look at the third lasso tool that Photoshop gives us for making selections, the Magnetic Lasso Tool!

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