Matrix
The tissue under the base of the nail where new nail is made.
Mostly hidden; the pale half-moon at the base is the visible end of it. Every cell of the plate starts here, so damage to the matrix shows up as a ridge, a split or a misshapen nail for as long as it takes to grow out — and if the damage is deep, permanently.
This is the reason a tech works lightly at the cuticle and why pressure from an e-file there is worth objecting to.
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