Semi-colons and colons are both used to signal relationships between ideas; however, the relationship signalled by each punctuation mark is different. Colons introduce explanatory information or information that is a restatement of the information preceding the colon; semi-colons introduce additional or contrary information.
The following examples illustrate these differences.
Colon used to introduce explanatory (or supportive) information
Working memory is limited in terms of capacity:
it can only process around seven items of information at any one time
| Here the colon is expressing a relationship between the two ideas that is equivalent to the connective word BECAUSE: introducing an explanation of the previous idea. |
Colon used to introduce information that is a restatement of the information
preceding the it
Working memory, in comparison to long term memory, is
limited in terms of capacity: it cannot process as much information
as long term memory.
| Here the colon is expressing a relationship between the two ideas that is equivalent to the connective phrase IN OTHER WORDS: restating the previous idea. |
Semi-colon used to introduce additional information
Working memory is limited in terms of capacity;
it is also limited in terms of duration.
| Here the semi-colon is expressing a relationship between the two ideas that is equivalent to the connective word AND: adding additional information. |
Semi-colon used to introduce contrary information
Working memory is limited in terms of capacity;
long term memory possesses an unlimited capacity.
| Here the semi-colon is expressing a relationship between the two ideas that is equivalent to the connective words BUT or IN CONTRAST: introducing an opposite idea. |
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