Lihit Lab Book-Style Pen Case

Lihit Lab is a Japanese stationery and office supply company that's been around since 1938. One of their high-quality products, this book-style pen case, allows you to store and organize all manner of stationery inside.

It features a three-sided zipper opening that allows it to open wide like a book, and has an inner "page" that divides the bag into two sides — one with straps for holding writing utensils (particularly those with clips):

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...and another with pouches for holding small items (erasers, sharpeners, small rulers and scissors, lead packs, etc):

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Each pen case is $9–$11 on Amazon depending on your color of choice: