Legal employers typically seek legal analysis in writing samples; therefore, a memorandum or brief is preferred over a research paper. The ultimate criterion, however, is the quality of the writing. If you use a document prepared for a prior employer, you must get the employer’s permission and make sure you have made all necessary modifications and redactions to preserve client confidentiality.
A cover sheet is useful to give any necessary background information about your writing sample. For example, if you use a writing project prepared for class, give the name of the class and a brief description of the assignment. If you are using a document prepared for a former employer, explain that you have obtained the employer’s permission and made all necessary modifications.
Although there is no definitive ideal length for a writing sample, 5-10 pages typically demonstrates your writing ability. If all of your potential writing samples are much longer, consider using an excerpt (e.g., one argument from a longer brief) and provide a brief explanatory note in the form of a cover sheet.