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Penfriend saves key-strokes by typing letters for you. This helps writers to get more on paper in the same time, and to write more before they get tired.
In a detailed academic study of supportive writing software by the CALL Centre, Penfriend was found to be the most effective word predictor. It saved around 65% of keystrokes (more than any other product), and predicted the correct word by the time the first letter had been typed in around 70% of cases (again, the best).