This letterbook covers the period June-September 1886. Most of the correspondence is by Samuel Insull. There are also letters by Alfred O. Tate. The book contains correspondence relating to the commercial development of Edison's phonoplex telegraph system in the United States and Canada. Included also is a list of phonoplex components sent to various offices of the Baltimore & Ohio Telegraph Co. Among the recipients are numerous railroad and telegraph companies including the Baltimore & Ohio Telegraph Co., the Canadian Pacific Railway, the Great Northwestern Telegraph Co., the Newport News & Mississippi Valley Telegraph Co., and the Pennsylvania Railroad Co. The front cover is labeled "June 23-Sept. 14, 1886 Phonoplex Correspondence. Previous letters were copied in the general letter books." The spine is labeled "1886" and is stamped "Letters." The book contains 301 numbered pages and an index. Approximately 60 percent of the book has been selected.