PowerPoint has two kinds of on-screen guides (dashed lines) to help you align objects on your slides while you are moving or sizing them: smart guides and static guides.
While you're moving or sizing objects (or cropping a picture) on your slide, smart guides automatically appear to help you fine-tune the layout. They appear when your objects (such as pictures, shapes, and so on) are close to aligned, and they also appear when objects are spaced almost evenly. The illustrations below show some examples of what smart guides can help you with:
By default, smart guides are turned on, but if they aren't on for you right now, turn them on in PowerPoint this way:
Turn on smart guides
In Normal view, right-click an empty area or margin of a slide (not a placeholder), and then select Grid and Guides.
The Grid and Guides dialog box appears.
In the lower half of the dialog box, under Guide Settings, select Display smart guides when shapes are aligned.
Turn off smart guides temporarily
Press the Alt key and hold it down while you are dragging an object.
Smart guides will turn on again after you release the Alt key.
For a video demonstration of smart guides, see Align and arrange objects on slides.
The default static guides consist of a centered vertical guideline and a centered horizontal guideline. The intersection of the two shows you the center of the slide.
You can add more static guidelines to help you arrange objects on your slides. You can also color-code the guides. Guides can help you arrange things consistently across an entire set of slides. See Use master-level guides to align objects across all slides for more information.