Retention is a coordinate.

A periodic structure earns its name when where a thing sits tells you how it behaves. For memory, the dimensions are how long the trace lives, how the substrate is refreshed, how many distinguishable states it holds, what it costs to write, hold, and recall, how the trace is reached, how it evolves while held, what form it takes, how it enters, and what guarantees travel with it on recall.

Read across the coordinate and the system explains itself — what it can retain, what it must rewrite, what survives a partition, what it pays per bit in joules. The same map covers a CPU register, an L1 cache line, a DRAM row, a flash page, a hippocampal engram, a cortical schema, a CRISPR spacer, a strand of DNA, a Vedic verse, an Aboriginal songline, an Inca khipu, and a single trapped ion.

The map is a way of seeing, not a verdict. It classifies what retains, makes the structure of the design space legible, and lets a retention substrate be reasoned about before it is built — the same move the sibling stacks make for computation, synthesis, data, state, and communication.