Number 2 pencils: What makes them so special?

It's a staple on everyone's back-to-school list, but why do we always use No. 2 pencils? They really are the best writing utensil for the job.
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They're at the top of every back-to-school list, but does anyone know why No. 2 pencils are so important?

What Makes a No. 2 Pencil Different?

Wood-cased pencils (150-count) for writing, drawing, or sketching; ideal for home, office, or classroom #2 HB medium-soft lead for strength, smooth

Basics Woodcased #2 Pencils, Pre-sharpened, HB Lead Bulk Box, 150 Count, Yellow

Favorable Colors Designs: these personalized motivational pencils are featured in solid color with inspirational sentences on the surfaces, assorted

20 Pcs Inspirational Pencils Compliment Pencils with Motivational Sayings #2 Pencils Back to School Pencils Cute Fun Pencils for Kids Girls Student

Number 2 pencils: What makes them so special?

Anything special about any of these three pencils? : r/pencils

11 Best Pencils for Writing and Erasing, Tested by Teachers

Ticonderoga My First Large Beginner No. 2 Pencils - #2 Lead - 10.3 mm Lead Diameter - Graphite Lead - Yellow Wood Barrel - 2 / Pack

Ticonderoga® My First® #2 Pencils with Sharpener, 5 ct - Baker's

pencil – Margret puts pen to paper

Grab Those Number Twos!

BIC Xtra-Sparkle No. 2 Mechanical Pencils with Erasers, Medium Point (0.7mm), 24 Pencils

no pencil, large reduction Save 86%

Some more old pencils I've found (don't know much about them so if you do please share) : r/pencils

Noted by Post-it®, Mechanical Pencils, Multicolor, 3 Pens/Pack