If content elements such headings and texts are too similar, the viewer will not be able to tell them apart.
Contrast means to make the elements very different, for instance dark text on light background, or large text next to small text. Contrast in poster design works with font bolds and styles, colour, and size.
Every element in your poster - text and images - should have a visual connection to another element.
Alignment has to do with lining things up in a consistent manner. For instance, earlier we said that your text in text boxes should be aligned left within the text box. This particular alignment expresses the connection with the borders of the text box and all other text boxes. If your text is centered within boxes, then all text will be uneven in all boxes. Similarly make sure that all columns are the same width and have the same distance (white space) between them. Make sure that the tops of boxes align across a poster. Microsoft Publisher has powerful tools to automatically align text.
Repetition is when a design element is repeated in your poster. It creates consistency.
For instance, if all your headings are bold 44 point font in pink boxes, your reader understands this visual grammar and uses it to help read your poster quickly. When rules of repetition are broken, it can make your poster visually chaotic or confusing. For instance, using different bullet point styles for different lists the poster will seem disorganized.