Fear has a smell, terror has a colour
The US Department of Homeland Security has a colour-coded system in place to tell Americans how threatened their country is from terrorists.
Each of the five colours - green, blue, yellow, orange and red - represents a level of threat, and recommendations are offered of the protective measures that should be taken at the various stages.
Green is the lowest level, which is declared when there is slight risk of terrorist attacks.
A blue, or guarded condition, is given when it is determined that there is a general risk of an attack.
A significant risk of an attack is represented by the yellow, or elevated, condition.
The signal raised in New York, Washington and New Jersey this week, orange, meant that a high risk of an attack by terrorists existed.
Red, the top level of alertness, means a severe risk of attacks exists. Government departments and agencies are expected to mobilise emergency teams and send staff to shelter areas.
The highest level of alertness reached since the scheme was introduced last year has been orange.