The Ultra Fine Black Sharpie works well to record sample string heights with Grandwork String Height Gauges (included when they are purchased). Its marks can be erased with alcohol, allowing plungers to be reused. Or it can make permanent Center Punch dimples more visible.
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Sharpie Ultra Fine Point Black Permanent Markers, 2/PK, (37161PP)
Proudly permanent ink marks on paper, plastic, metal, and most other surfaces Intensely brilliant colors create eye-popping, vibrant impressions
SHARPIE Retractable Permanent Markers, Ultra Fine Point, Black, 3 Count