New theoretical curves relating the hysteresis parameters Mrs/Ms and Hcr/Hc for single-domain (SD), superparamagnetic (SP), pseudo-single-domain (PSD), and multidomain (MD) grains and their mixtures are applied to published data for natural materials. The Day plot of Mrs/Ms versus Hcr/Hc has been used to crudely classify samples into box-like SD, PSD, and MD (or sometimes incorrectly, MD + SP) regions with arbitrary boundaries. New type curves for MD, PSD/SD + MD, and SD + SP grains and mixtures permit more subtle and precise modeling. The predicted MD trend and its junction with the PSD trend are observed in two data sets: for magnetite spherules from carbonate rocks and for temperature-varying hysteresis results spanning the Verwey transition. The latter data are the basis of a suggested new method for pinpointing the PSD-MD threshold size. Selected data for pottery clays, soils, and paleosols generally follow SD + MD type curves and indicate intermediate-size PSD magnetite with narrow to broad size distributions. A lake sediment section with known grain-size progression tracks in the predicted sense along the SD + MD trend. Selected data for glaciomarine and pelagic sediments are also generally compatible with SD + MD trends. Examples of remagnetized carbonate rocks, submarine basaltic glasses, and glassy rims of pillow basalts all follow predicted SP + SD or SP + PSD mixing curves, with a large range in volume fraction of SP grains (0--75%) but a narrow range of SP particle sizes: 10 ¿ 2 nm. Larger SP grains spanning the range to SD size (25--30 nm) are absent for unknown reasons. Oceanic dolerites, gabbros, and serpentinized peridotites in some cases fall in a novel region of the Day plot, parallel to but below magnetite SD + MD mixing curves.