The quality of being customary; the degree to which something follows established tradition or common practice.
From 'customary' plus the suffix '-ness' (state or quality). This creates an abstract noun describing how traditional or habitual something is, though the word itself is rarely used.
While 'customariness' is grammatically perfect, native speakers almost never say it—we say 'it's customary' instead, showing how English prefers adjective + 'is' over abstract nouns, and revealing the natural path languages take toward efficiency.
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