Proofreading is an important last step in the writing process. Both technical writers and editors need to be careful proofreaders, both of their own work and that of others. If work is full of grammatical errors or typos that should have been caught earlier in the publishing process, then even if the writing is technically accurate and well-researched, it loses its credibility with the reader.
Although true proofreading is really meant to happen once the document has come back from layout and is in its final form (a final check of the proofs), I think it is important to know and use the principles of proofreading after a document has been copy edited and the changes have been incorporated. Even if there is not enough time to do multiple passes of the document, a final proofread will be very useful in catching errors.
The Chicago Manual of Style is a good reference to consult for learning the common proofreading marks. I think these marks are very useful to know — they really are universal symbols that should be understood by all proofreaders. Here is a link to its site:
My next post will discuss some helpful proofreading techniques.