And it is extremely insulting to point the sole of one's foot at another person, even if it is done unintentionally while sitting and chatting informally.

NPR: Sergeant Notes Odd Rituals of War in Afghanistan

Without standing, he lifts his left foot and drags a brittle friction match across the sole of his boot, then watches the red tip flare and expire in the chill wind before he can transfer the flame.

NPR: Excerpt: 'Woodsburner'

The English foot, for example, is almost identical to the Japanese Kanejaku, and both are as long as the sole of an average man's shoe.

BBC: Gonna take a centi-mental journey