# C243 — The 6875 interval center is one year earlier than the 6877 center

23 September 2026 · Step 38 of 60, continuing C205.

Because the left endpoint is held fixed, shortening the interval from 6877 to 6875 moves its center one year earlier. The right endpoints remain two years apart in both components.

| p | Double-11 center | Initial-13 center | Center difference |
|---|---|---|---|
| 3 | -1963/2 | -1961/2 | 1 |
| 5 | -1959/2 | -1957/2 | 1 |

This is not a no-year-zero correction, a seasonal ambiguity or a one-year Flood start/close change. In these two primary rows the right endpoint changes by a full two-year Flood Gear step. The distinction is essential before the full native field, including close rows, is tested.

## Evidence and scope

K=529 and r=25/23. BC base B means (B+1)t/Bn BC; reflected AD base A means AD At/(A+1)n. Their civil span is B+A in both seasonal components, with no year zero. Signed paired centers are −B and +A; the center model uses components ±1/4 and restores this separation after dilation. An interval center is geometry, not an event or a proposed universal Mirror axis.

Regular Gears are restricted to Noah/Shem/Flood. Creation and Conquest retain fixed source-field members; no fourth Gear is admitted. Generated coordinates, source-field membership and stored graph incidence are distinct. File_18/File_09 and the frozen C127 graph are unchanged. Earlier N01/N03 qualifications, the missing Shem join, closed primers and unestablished original Reports 11–25 remain unchanged. This packet selectively uses prior evidence; it does not rerun completed campaigns. Assertions and alternative placements are not independent historical witness counts.

**6 step-specific checks and 19 frozen-input checks passed.** Exact inputs, results and predecessor hashes are in the accompanying JSON records; calculation code is in the evidence archive. Source dependencies are identified in the body and frozen in `sources/`.