In 1723 Edmund Stone published The construction and principal uses of mathematical instruments, which was essentially a translation from the French of Bion’s Traité de la construction et des principaux usages des instrumens de mathématique(1709). As the title of the book indicated, Stone annexed a number of instruments that had been omitted by Bion, in particular, those invented or improved by theEnglish. Hence, after the translation of Book II, on the construction and uses of the « French sector », Stone added a chapter on the « English sector ». In the 17th century there had been a number of debates concerning the amount of mathematical training required for the study of mathematical instruments. In the context of the study of mathematical instruments in the 18th century, it is worth exploring the link theory-practice in the books on instruments. The aim of this contribution is to explore the mathematical knowledge involved in the use and applications of the « English sector » in trigonometry in a number of 18th-century books on mathematical instruments.