甲

• Hard outer casings: includes tortoise shells (specifically those used as a medium for pyromancy), protective plating or breastplates (armor), and biological nails (fingernails).
• Ordinal priority: the first of the ten heavenly stems (//tiāngān//); by extension, denotes "chief," "leader," or superiority in rank and quality.

• Noun: used for physical materials (shell, armor, nail).
• Temporal marker: functions as an ordinal marker in dates and the sexagenary cycle (e.g., //jiǎzǐ// 甲子).
• Classifier/Adjective: used to indicate rank or grade (e.g., "first-rate").

• Readings: Fanqie reading 古狎; Tang-period reading //gap//.
• Early usage: In oracle bone inscriptions, specifically refers to the tortoise shell.

In military or administrative texts, be alert for potential ambiguity between the material and systemic domains: a reference to 甲 may denote physical armor (an armored unit) or ordinal priority (the primary/first unit).