If you have the grain going the wrong way the paper will ripple, the fold will not be clean and the book will want to close itself.

如果把纸的纹路弄反了，纸就会起皱，折边也不整齐，书还总是会自己合上。

youdao

Fold the letter AD put it into the envelope and then close it.

把信折叠起来并且把它放进信封并且关上信封。

youdao

It has close relationship with racial difference. It exists in Mongolian inCentral Asia, North Asia and East Asia mostly, so it is also called "mongoloid fold".

内眦赘皮与种族差异有着密切的联系，最常见于中亚、北亚、东亚等地区的蒙古人种中，故又称“蒙古皱襞”。

youdao

Still in concept stage, the steering wheel features two handles that close fully and fold away into the dashboard, which the company says significantly improves driver comfort and accessibility.

FORBES: Ready for a Folding Steering Wheel?

At the time of writing, all the major Euro countries are in crisis, with Germany close to joining the fold, with growth set to halve next year (assuming Euro-zone problems get no worse).

FORBES: Congratulations, But Can The World Outside Europe Live With The EU and Euro?

That's close to a four-fold increase over the previous year, 2004.

NPR: Iran Most Active in Terrorism, Says State Dept.