The “junior high school” concept was introduced in 1909, in Columbus, Ohio. Junior high schools were created for "bridging the gap between the elementary and the high school", an emphasis credited to Charles W. Eliot. In the late 19th century and early 20th century, most American elementary schools had grades 1 through 8, and this organization still exists, where some concepts of middle school organization have been adapted to the intermediate grades. As time passed, the junior high school concept increased quickly as new school districts proliferated, or systems modernized buildings and curricula. This expansion continued through the 1960s. Jon Wiles, author of "Developing Successful K–8 Schools: A Principal's Guide", said that "[a] major problem" for the original model was "the inclusion of the ninth grade", because of the lack of instructional flexibility, due to the requirement of having to earn high school credits in the ninth grade and that "the fully adolescent ninth grader in junior high school did not seem to belong with the students experiencing the onset of puberty".