With the Eratosthenes Sieve, the even numbers "cloud" the perspective of the pattern. Using only the odd numbers, the pattern is a little more evident. From 21 and 25 there is 27, 33, 35. the 3 composites move closer to the five composites by one odd number. With two more 3 composites, 39 and 45, the 3 composites meet the 5 composites. 7^2 jumps in at 49, but doesn't upset the 3,5 dance until 63, where 3 and 7 meet. The composite numbers 51,53,57 mimic the pattern of 21,25,27. 63,65,69 copy the pattern of 33,35,39 only because 3 and 7 meet. 75, where 3 and 5 meet again (every 15 odd numbers) and 77 show the 7 composites running away. 81,85,87 show only the 3,5 pattern. 91,93,95,99 shows the 7 composite jump into the 3,5 pattern again. At 105, 3,5,7 meet to form a composite. Then it starts over.3 odd numbers (ON) from 105 is 111. 5 ON from 105 is 115. 6 ON from 105is 117. 7 ON from 105 is 119. 11^2 enters at 121, but is only relevant every 22 whole numbers, or 11 ON. (121+22=143)(123, 125, 127, 129, 131, 133, 135, 137, 139, 141, 143. 11ON from 121 to 143)