This is when things are grouped together and they are related. When things are not organized and grouped together it can be confusing and lack unity. This organization can over power color.
Alignment helps organize design also. It makes things more uniform and cohesive. What you are trying to convey will be stronger with alignment- like a year book. Alignment has implied and actual lines.
"Repeat some aspect of the design throughout the entire piece."
This can used in color, alignment, or an element that repeats.
this makes things stand out from one another
dark vs. light
thick vs. thin
tall vs. short