1 stop is the difference between 200 and 400. One more stop than 400 would be 800. One less stop than 200 would be 100. Everything in photography is half and double. Twice 200 is 400, half 200 is 100. Both are 1 stop. Twice 5.6 is 8. Half 5.6 is 4. Both are 1 stop. Twice 1/125 is 1/250. Half is 1/60. Both are one stop. All three legs make the stool. Capiche?