There are a few rules about shutter design such as louver upstairs and panel downstairs but simple and complex shutters have always had their place. The simplest of shutters are Board & Batten consisting of planks held vertically buy cross braces called battens. These "Board & Batten" shutters are about as simple as they get but are appropriate to most window openings. The planks or 'boards' generally have a gap between them that serves to vent the house without offering too much view for snooping eyes.
Complex shutters have the decorative raised panels of varied profiles with select trims and numerous cross rails to mimic window / transom layouts. Of all the raised panel designs, only flat and edge angled designs were available until the turn of the 17th century when shapers were available for more complex decorative shapes. It's often said that, "... a house without shutters is like a person without eyebrows". So, choose your eyebrows wisely!