When the behaviors of a stutter are infrequent, brief, and are not accompanied by substantial avoidance behavior, the stutter is usually classified as a mild or a non-chronic stutter. Non-chronic stuttering is often called "situational stuttering" because the afflicted person generally has difficulty speaking only in isolated situations—usually during public speaking or other stressful activities—and outside of these situations the person generally does not stutter. When the behaviors are frequent, long in duration, or when there are visible signs of struggle and avoidance behavior, the stutter is classified as a severe or chronic stutter. Unlike mild or situational stuttering, chronic stuttering is present in most situations, but can be either exacerbated or eased depending on different conditions
Severe stutters often, but not always, are accompanied by strong feelings and emotions in reaction to the problem such as anxiety, shame, fear, self-hatred, etc. This is usually less present in mild stutterers and serves as another criteria by which to define stutters as mild or severe. Another way of discerning between the two severities is by percentage of disfluency per 100 words. When a speaker experiences disfluencies at a rate around 10%, they usually have a mild stutter, while 15% or more is usually indicative of a severe stutter.
In addition to the disfluency, many people who stutter display secondary motor behaviors. Observers often notice muscles tensing up, facial and neck tics, excessive eye blinking, and lip and tongue tremors. In extreme cases entire body movements may accompany stuttering. Most common with stutterers is the inability to maintain eye contact with the listener, which in many cultures may hamper the growth of personal or professional relationships.
It is worth noting that the severity of a stutter is not constant and that stutterers often go through weeks or months of substantially increased or decreased fluency. Stutterers universally report having "good days" and "bad days" and report dramatically increased or decreased fluency in specific situations. Below is an overview of the circumstances that harm and help the fluency of most stutterers.