continual vs. continuous

Most usage guides make the following distinction between these two terms.

Continual is most often used to mean "constantly or frequently repeated":

• Reading was a continual source of enjoyment for Rashid during his long recovery.
• A continual honking of horns sounded from the cars in the wedding procession.

Continuous means "uninterrupted" (over distance or time):

• A continuous expanse of sand stretched to the horizon, unbroken by any vegetation.
• The continuous flow of water from the broken pipe soon flooded the basement.

Be careful not to use continuous (uninterrupted) when you mean continual (constantly repeated):

• Sadie's continual (not continuous) complaining began to get on her co-workers' nerves.

Sadie might have complained constantly—but she couldn't have complained without interruption (she must have spent some time working, and no doubt she went home at night). Therefore, continual is the word needed here.

In Garner's Modern American Usage, Bryan Garner suggests this handy trick to remember that continuous means "without interruption": think of the ending -ous as standing for "one uninterrupted sequence."