Markers, frames & measurement¶

Scrubbing the playback and judging position or angle by eye only gets you so far. Two tools make measurement exact:

A Marker is a point in space with no mass and no motion of its own — it exists purely so you have somewhere to measure from or to. Place one on a body (it then moves with the body, at a chosen offset) or at a fixed absolute location.
You don't always need a Marker: for a fixed point in empty space, a Request can anchor directly to a ground point (just X/Y/Z), no Marker required. Markers earn their keep when the point you care about isn't a body's own node — e.g. an offset point on a swinging body.
A Request (Distance, Velocity, Acceleration, Force, Angle) samples a quantity every frame, between two such anchors, or from a joint. Its dialog shows a live "Current: … @ frame N" readout that updates as you scrub — an exact numeric check, not an eyeballed one.

Together, Markers and Requests turn "did it move correctly?" into a number you can compare against a closed-form result — see Validating a model.