Schematic showing the tethered particle method. (a) The tether is attached to a specific point on the bead; *z* is the height of this point. **r** denotes the position of the center of the bead. (b) The vector **R** from the attachment point to the bead center can rotate and is described by two angles. These rotations are more constrained for small values of *z*. Note that in the figure the width of DNA is not to scale, it is much smaller in real experiments.