duž

1. (+ gen.) along
   Šetali smo duž obale do mraka. — We walked along the shore until dark.
   Duž ulice su parkirani automobili. — Cars are parked along the street.

2. (in geometry) line segment
   Nacrtaj duž AB dugu pet centimetara. — Draw a line segment AB five centimetres long.

The preposition duž takes the genitive: duž reke 'along the river'.