THE DOOMSDAY RULE
the weekday of any date in the Gregorian calendar, worked out in the head in three steps
LEAP YEARS
one rule, a cycle of four hundred years and the ninety-seven extra days in it
THE RULE OF 72
the time it takes steady growth to double: the division you can do in your head, how far it misses and why the divisor is what it is
EQUAL TEMPERAMENT
how many cents each interval misses the pure one by, and why exactly that much
TONEWOODS
density, hardness, export · what is measured kept apart from what is taste
THE COMPASS ROSE
the sixteen compass points with exact bearings: four cardinal, four intercardinal, eight in between
USDA HARDINESS ZONES
the 13 zones by average annual extreme minimum — each range in °F and °C, with the a and b half-zones
THE MOMENT MAGNITUDE SCALE
what a single step hides: ×10 in recorded amplitude and roughly ×32 in released energy — drawn to true scale
PLASTIC RESIN CODES
the seven codes on the bottom of an article per ASTM D7611: which resin each number names and what it means for sorting
SPEED THRESHOLDS
six thresholds where a different physics begins · every figure computed, not typed
MORSE CODE
26 letters and 10 digits · dot, dash
THE BASKETBALL
four FIBA sizes with their circumference and mass — and why the rules set a bounce, not a pressure