In September 1967, Owen Maynard of the Manned Spacecraft Center in Houston, Texas proposed a series of Apollo missions that would lead up to a manned lunar landing. Seven mission types were outlined, each testing a specific set of components and tasks, and each previous step would need to be completed successfully before the next mission type could be undertaken. These were:
D - Manned CSM and LM development in low Earth orbit (originally planned for Apollo 8; flown as Apollo 9)
E - Manned CSM and LM operations, a simulated lunar mission in an elliptical medium Earth orbit with an apogee of 3,500 nautical miles (6,500 km)), never flown
The planned sequence was changed when it became clear the first manned Lunar Module, LM-3, would not be ready in time for the December 1968 launch date of Apollo 8, so Apollo 8 was flown as a lunar orbital mission using just the CSM (sometimes referred to as "the C Prime mission") and the E mission was canceled.
F - Manned CSM and LM operations in lunar orbit, a "dress rehearsal" for the first landing (Apollo 10)
I - long duration CSM lunar orbital surveys using a Scientific Instrument Module mounted in an empty Service Module bay. These were incorporated into the J missions.
J - longer three-day stays using an Extended LM, with three LEVAs and a Lunar Roving Vehicle (Apollo 15, Apollo 16, Apollo 17). Apollo 18 to 20 would have been J missions. Apollo 15 was originally planned as an H mission but was promoted to J as the program was curtailed.