Textbook of Usability

A signifier, in Don Norman's later terminology, is a perceivable indicator that communicates an affordance — a visual, auditory, or tactile cue that tells the user where to act and what to do. Signifiers are distinct from affordances themselves: an affordance is a possibility for action; a signifier is evidence of that possibility.

Norman made this distinction because many interface "affordances" are in fact signifiers — visual conventions that suggest interaction but don't physically enable it. Pixels cannot actually be pressed; a button graphic is a signifier for the action of clicking.

Common signifiers:

• Button labels and raised appearance signify that something is clickable
• Underlined text in a link colour signifies navigation
• Arrows on scroll bars signify scrolling direction
• Cursor shape changes signify draggable elements or hover targets
• Physical handles, indentations, grip patterns on real objects
• Signs and labels in physical environments

Norman argues that signifiers are often more important than true affordances in design, because signifiers actively communicate the designer's intent. An interface without signifiers may be technically operable but will leave users guessing. Flat design — which reduces visual cues — has made signifier loss a common usability issue in modern interfaces.