Three 147 rails require two source conditions
The three 147-year rails measure different relationships: Isaac’s birth to Levi’s birth, Jacob’s own lifespan, and Jacob’s call to Levi’s death. Their agreement can be reduced to two source conditions. The first is 60+77+10=147: Isaac’s interval to Jacob, Jacob’s call age, and the interval from that call to Levi’s birth. The second is 10+137=147: the same household interval followed by Levi’s lifespan.
The familiar vertical separations of 60 and 77 then follow from these two conditions. Subtracting one condition from the other also gives 60+77=137, explaining the shared diagonal. These further equalities describe the same connected arrangement rather than adding separate source requirements.
Ishmael’s and Levi’s equal 137-year lives supply another premise. Equal lives preserve their birth separation at death, producing the corresponding 161 displacement. The further 161 years from Levi’s death to the terminal still depends on the terminal’s placement.
This distinction helps the families fit together: some agreements come from shared paths, some from equal-valued biographies, and others from an independently retained endpoint. Their explanatory roles are different even when the displayed numbers agree.