No key, no chart, nobody to teach you
The shadow does not stand for the time. The shadow is the time, moved by the same sun that makes the day. Hotter air makes taller mercury. A shaking pen draws a shaking line. In each case the record and the thing are the same shape.
Which means a stranger can read them without being told anything. Hand a thermometer to somebody who has never seen one and they will work it out in a minute. Hand them a page of morse and they will not, however clever they are.
A footprint is the oldest one. Nobody designed it, nobody agreed it, and it still tells you the size of the animal, which way it went and how long ago.
And why none of them can keep a secret
The very thing that makes these easy also makes them useless for hiding: if the resemblance is there, anyone who notices it can read the record. There is no rule to keep private, because there is no rule at all.
That is the line between this branch and the ciphers. A code by agreement can be private, because the agreement can be. A code by resemblance never can.
The record groove Codes made of position The ciphers