16mm vs. Normal 8 (aka Double 8)
This image above shows the difference between two different types of 16mm film perforation holes.
Shown from left to right (note that all perforation holes are the same size):
1) regular 16mm film - used in a 16mm camera
2) 16mm wide "Double 8" film (which was 25 feet long, when the unprocessed roll was purchased) - used in an 8mm camera
3) 8mm film "Normal 8" - split lengthwise (During processing at the lab, the lab would split the 25 foot reel into 2 sections of 25 feet, then joined the two 25-foot lenghts. That reel became a 50 foot reel!)