[1]: 94  When defined very broadly, the group consists of plants with dichotomously branched, naked aerial axes ("stems") with terminal spore-bearing structures (sporangia).

Edwards: "plants with smooth axes, lacking well-defined spines or leaves, showing a variety of branching patterns that may be isotomous, anisotomous, pseudomonopodial or adventitious.

[6] Taylor et al. in their book Paleobotany use Rhyniophyta as a formal taxon,[3]: 1028  but with a loose definition: plants "characterized by dichotomously branched, naked aerial axes with terminal sporangia".

Rhynialeans (order Rhyniales), such as Rhynia gwynne-vaughanii, Stockmansella and Huvenia, had radially symmetrical sporangia that were longer than wide and possessed vascular tissue with S-type tracheids.

Cooksonioids, such as Cooksonia pertoni, C. paranensis and C. hemisphaerica, had radially symmetrical or trumpet-shaped sporangia, without clear evidence of vascular tissue.

[1]: 96–97  The following are some of the names which may be used: In 2004, Crane et al. published a cladogram for the polysporangiophytes in which the Rhyniaceae are shown as the sister group of all other tracheophytes (vascular plants).

[8]: 329 It has been suggested that the poorly preserved Eohostimella, found in deposits of Early Silurian age (Llandovery, around 440 to 430 million years ago), may also be a rhyniophyte.

[12] The general term "rhyniophytes" or "rhyniophytoids" is sometimes used for the assemblage of plants found in the Rhynie chert Lagerstätte - rich fossil beds in Aberdeenshire, Scotland, and roughly coeval sites with similar flora.