There are all sorts of reasons you might want to be able to find all of the vertical (portrait) images in a particular batch. Or all of the horizontal (landscape) images. Or all of the square images.
Maybe you need something for a magazine cover. Or for a social media header banner. Or an image to fit in a collage of square tiles.
Whatever your reason, Lightroom has an easy way to filter selections to isolate vertical, horizontal, or square images.
The filter in this case is called "aspect ratio." But the way in which "aspect ratio" is used in this context isn't necessarily self-evident. Aspect ratio to me usually brings to mind things like 3:2, 4:3, 16:9, or square 1:1. Characteristics like whether it's vertical (portrait) or horizontal (landscape) are more what I'd consider to be orientation.
That's not to stay that the usage is wrong here--it's not. Especially since one of the options is to find square images, which is a true aspect ratio rather than which directly it's lying. It's just not the term that immediately comes to my mind.
Another thing worth knowing is that this is an imperfect filter that doesn't catch everything. You might also come across photos showing up as "unknown" aspect ratio. There are a few different ways that images might fall into the unknown aspect ratio, because Lightroom is using a field in the image's metadata rather than analyzing for itself. Unknown aspect ratio files might be video files or scans or something captured with a camera or device that doesn't save the orientation in the image metadata.
In the Library module, select the batch of images you want to filter. That might in a collection, smart collection, or library folder.
Open the filters panel. The quickest way to do this is to press the backslash key (). Or you can use the menu item View > Show Filter Bar.
Scroll down the list of options to find Aspect Ratio.
Once you activate that filter, you'll see a short list of the aspect ratios that have been detected in the selected images. The options are Portrait, Landscape, Square, and Unknown. But you'll only see options that are relevant to the selected images. That is, if there are no square images in your selection, you won't see Square on the list.
As with any of the other filters, you can combine them with other filters as well. So, for example, you could choose all of the vertical images that don't have GPS data, or all of the vertical images taken with a particular camera or lens or at a certain aperture. Or only a specific filetype. And so on.
Saving Aspect Ratio as a Quick Filter
Once you've set the filter criteria to your liking, you can then save that as a filter shortcut for next time. For instance, if you find yourself often need just portrait orientation images, you can save that as a shortcut.
To do that, set your criteria however you like it. You can mix and match any of the usual filter criteria.
Then go to the small drop-down menu at the right-hand side of the filter bar. It should say "Custom Filter" or an abbreviated version if space in the view screen is tight.
When you click on that, you'll get a drop-down menu. There are some preset shortcuts built in, so you'll probably see a short list there. (You can also clear out the default list, in which case there might not be any presets already pre-populated--which is why I mention it.)
Scroll down to the bottom of that list for the options to save new presets or update existing ones. You want "Save Current Settings as New Preset."
You'll then get a popup to assign a name for the preset.
And with that, the preset is saved. When you want to use it as a shortcut, there are two places you can do so. One is using that same drop-down menu from the top filter bar.
There's a second option that doesn't require you to open the top filter bar at all. And that is to use the drop-down menu at the bottom right of the interface, next to the flag, color, and start pickers.