This depends on what kind of climbing you mean. Are we talking local crags, boulder gyms, climbing walls, multi-pitch climbs or crazy alpine trips? Climbing is generally best in trousers made from tough, hardwearing fabrics, like outdoor trousers, hiking pants or good pants for outdoor work.
For the climbing gym and summer rock climbing, thinner stretch fabrics make the best materials for climbing pants and many people prefer to climb in shorts. As soon as you head outdoors, it’s also good to have some protection in cooler temperatures and light winds.
For sports climbing and bouldering in particular, you often see pants with elasticated waists or drawstring waistbands with a low profile. Both versions work particularly well under a harness. When it comes to patterning, articulated knees and gusset inserts also give you additional freedom of movement.