Wiley Nautical carry a title list of practical guides and educational books. This new practical instruction is well up to their established standard and features colour throughout, with copious illustration using drawings, sketches and photographs. A great many sailors never really learn how to trim their sails. Some, like this reviewer, will start sailing at a young age and without formal instruction, learning by experience and suiting their sailing to the conditions and opportunities that they have available to them. Even small dinghies have benefited from the development of new technologies and now have a full range of controls that can be used to squeeze extra performance form their craft. This sophistication makes the learning process potentially much more difficult. Felix Marks has taken an interesting and effective approach by ruthlessly concentrating on the mainsail to the exclusion of other sails and unrelated rigging. The result is a very easy to follow review of the principles of mainsail design, manufacture and use.