How can I create a Powerpoint VBA Macro that specifically moves only the Rectangles to the background?

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Here's a VBA macro that moves all rectangles to the background in PowerPoint:

Sub MoveRectanglesToBackground()
    Dim shp As Shape

    For Each shp In ActivePresentation.Slides(1).Shapes 'change slide number as per your need
        If shp.Type = msoShapeRectangle Then
            shp.ZOrder msoSendToBack
        End If
    Next shp

End Sub

This macro loops through all the shapes on the first slide of the presentation and checks if it is a rectangle. If it is, then it sends the shape to the back of the slide.

Note: This macro will only work for shapes that are strictly rectangles. If you have other shapes that have a rectangular outline or that overlap with rectangles, they may also be sent to the back.