Variations of annual values of the signed open solar flux F_S, from various sources, showing how modern values (from magnetogram data via PFSS modelling and from the OMNI dataset of near-Earth interplanetary observations, panel a) are extrapolated back in time using geomagnetic observations (panel b), models based on sunspot numbers (panels c and d) and cosmogenic isotopes (panel d), Sunspot number sequences used as input to the models are shown in part e. Even-/odd-numbered solar cycles are shaded white and grey and the Maunder minimum is shaded pink. (See text for further details).