From The Airport Wiki

The Boeing 727 is a mid-size, narrow-body, three-engine commercial jet airliner. It first flew in 1963 and for over a decade was the most produced commercial jet airliner in the world. 1,831 727s were delivered. The 727's sales record for the most jets bought in history was broken in the early 1990s by the Boeing 737. In August 2006, a total of 127 Boeing 727-100 aircraft and 493 Boeing 727-200 aircraft were in airline service.[1]

The 727's fuselage has an outer diameter of 148 inches.[2] This allows six-abreast seating (three per side) and a single central access walkway when coach-class (18 inch width) seats are installed.