Series guideBefore you climb a crack, work out how wide it is: tips, fingers, off-fingers, hands, fists, offwidth, squeeze or chimney. Each size has its own jams. But you measure the size with your own hand. Pete Whittaker's comparison: cam sizes 0.75 and 1 are perfect hand jams for small hands and thin, strenuous jams for big ones; a #4 is a fist jam for big hands and a butterfly stack for small ones. So 'this one's easy' from a friend may not apply to you, and you have to climb every size for the difficulty to even out. The series starts with hand cracks, the easiest to learn, and works outward toward narrower (fingers) and wider (fists, offwidths), with 2020-or-later videos from many crags and climbers in every post.
A hand jam's strength comes from the thumb, not the fingers: relax and flatten the hand, slide it in to the wrist, then tuck the thumb into the palm and push it toward the little finger so the muscle at its base presses on the rock. The fingers only help. The most common mistakes are not going deep enough, grabbing the crack edge with your fingers, and jamming at chest height. Thumbs up to move up, thumbs down to rest or place gear; cross hands in straight cracks, shuffle in leaning cracks and corners. A crack that's perfect for big hands may be cupped hands for small ones, so find your own 'hand number' first.
In a hand crack a foot jam takes four steps: drop the knee outward so the foot turns to its thinnest side (little toe down), slide the toes in, then bring the knee back in line with the crack so the foot twists tight. The most common mistake is getting only the tips of the toes in. Sources disagree on depth; Pete Whittaker's sweet spot puts the crack edge half to a full toe's length past the webbing of your toes. As you weight it, let the heel sit lower than the toes: pressure moves off the toe knuckles and the jam gets more secure. Foot pain usually means over-twisting or shoes that are too tight. To get out, reverse it: knee out, ankle relaxed. Finger cracks take only the toe; fist cracks take the whole foot straight in.
Read a finger crack by how deep your fingers go: only to the first knuckle is tips, to the second knuckle or the base of the fingers is a good finger crack, and in but wobbling is rattly fingers. For the good size, go thumb down: fingers in to the base, palm and forearm flat on the rock, rotate the elbow down until the forearm lines up with the crack, and pull down, not out. Too thin, flip to pinkie down; rattly, rely on torque and push your thumb on the edge; any wider is off-fingers. Knuckles locked above a constriction cost the least effort, and pin scars are ready-made constrictions, though they are hard to protect and hard on your toes. Feet go on face holds first, then toe jams or front-on smears on the crack edges. If the jam won't stick, the edge is good to grip and the wall isn't steep, layback. A millimeter changes everything, so measure cracks with your own fingers.