It's a watch pocket, originally for men who wore pocket watches and needed a protective place to store them.

这其实是一个表袋，最初是为有怀表的人设计的，怀表放在这里面刚刚好。

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The Levi Strauss blog confirms their conclusion: "the first blue jeans had four pockets-only one in back and, in the front, two plus the small, watch pocket."

李·维斯创始人李维·史特·劳斯在博客中给出了最终答案：“最开始的蓝色牛仔裤有四个口袋，背面一个，前面两个，外加一个迷你表袋。”

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She had put her watch in her pocket for safe keeping.

为安全起见，她把手表放进了衣袋。

《牛津词典》

In a way, Journe's life as a watch designer started with his first tourbillon, a pocket watch version that he completed in 1982 when he was 25.

FORBES: Magazine Article

On the back of the watch, large rubies set in sunk gold chatons pop against Geneva waves that evoke the styling of an antique pocket watch.

FORBES: Surprise! The Latest Watch From MB&F Looks (Almost) Normal

Everything was relegated to the beauty of the pocket watch, contemporary sculpture and centuries-old mechanics.

FORBES: Surprise! The Latest Watch From MB&F Looks (Almost) Normal