Engineering notation is a version of scientific notation in which the exponent 
in expressions of the form 
is chosen to always be divisible by 3. Numbers of
forms such as 
,
,
340, and 
therefore correspond to engineering notation, while numbers such as 
, 
, and 
do not.
16 of the 20 SI prefixes (excluding centi-, deci-, deka-, and hecto-) correspond to engineering notation.
Engineering notation is sometimes written using "E notation," which represents for example 
as 3.0E9 or 3.0e-9. Here, the E or e is shorthand for 
and should not be confused with the base of the natural
logarithm 
.
Care must be taken when attempting to infer significant digits from numbers expressed in engineering notation. For example, while 
unambiguously indicates
two significant digits in scientific notation,
the number of significant digits in 
is not clear, with the only obvious way to indicate
significance being to write the number in the awkward form 
.
