Think of the colors in your viewfinder as if they're objects.
The colors can have forms.
The visual elements are the lines, shapes, tones, colors, objects, and subjects in your photograph.
Odd numbers of visual elements tend to be more pleasing than even numbers.
Things need physical support in a picture.
Avoid cropping a leg of a table, for example.
Otherwise, it will look like it is about to tip over.
You may need to use visual support.
For example, let's say you can't get an entire bridge in your viewfinder.
If you take the picture, the bridge will have support on one side of the picture but not the other.
However, if you take the photograph when a sailboat is under the unsupported part of the bridge, the boat will lend visual support to the bridge.
Visual support can be provide by less tangible factors, such as a darkening at the bottom of a photograph.
Words and faces in your pictures may distract from the image.
Some photographers take pride in never cropping.
Other photographers don't pay attention to any such strictures.
Avoid cropping people at their joints.
Cropping tops of heads is a matter of taste.
Some photographers don't, while others do.