A leader–follower arm pair is a joint-space teleoperation device. The operator physically moves the leader, an arm with the same kinematic structure as the robot, and the follower is commanded to match the leader's joint positions in real time. Because joints map one to one, no inverse kinematics is needed, and the operator directly controls the arm's full configuration, including wrist orientation.
The design became standard in low-cost robot learning with ALOHA, described by Zhao et al. (2023): "The user teleoperates by backdriving the leader robots, with the follower robots mirroring the motion." GELLO (Wu et al., 2023) generalized it to industrial arms, building 3D-printed controllers with "the same kinematic structure as the target arm".
In this matrix, the SO-100, SO-101 and Koch arms are used as leader–follower pairs in LeRobot, and bi-SO is two pairs. On the SO-101, the follower uses six STS3215 servos with 1/345 gearing, while the leader mixes 1/191, 1/345 and 1/147 ratios so that it can hold its own weight yet "be moved without requiring much force". LeRobot's HIL-SERL guide notes that the SO-101 leader's gearing lets it track the follower during exploration, which makes human takeover smoother than with the SO-100.
Two misconceptions recur. A leader is not plug-and-play with any follower: joint zero positions, directions and limits must be calibrated so that leader readings map to follower targets, and LeRobot stores a calibration per device id. And a passive leader records only what the operator does; it provides no force feedback, so contact-rich demonstrations rely on the operator's vision.