The paper examines the structure of the mathematical instrument making trade in London from the mid-sixteenth century to the opening of the Hanoverian era. This analysis of the trade is primarily based on evidence drawn from contemporary advertising. A distinction between informal editorial recommendations and advertising per se is made. It is concluded that up to the mid-seventeenth century mathematical instrument makers worked in either wood or metal. After that date a growing number of workshops advertised that they manufactured in (...) all media. Advertising was aimed at informing professional users from whom particular instruments could be purchased, but not on informing customers in specific terms of the range of instruments manufactured. It is concluded that until the early eighteenth century most mathematical instruments were commissioned. Only towards the end of the period is there evidence of over-the-counter sales, and advertising aimed at encouraging the growing consumer market to buy mathematical instruments for the practice of science as a social or recreational activity. (shrink)