This is the 3382nd most frequent Japanese word.
極細
"Extremely fine" or "ultra-thin."
極細 highlights the delicateness of the lines and the precision they require.
極細の線を引くためには特別な道具が必要です。
Special tools are needed to draw ultra-fine lines.
極細 is used here to describe the thinness of the pen tip, emphasizing its precision.
このペンの先は極細です。
The tip of this pen is ultra-fine.
極細 is used to describe the needles, conveying their suitability for detailed work.