Rhinanthus is parasitic on grass and the plant is named for its seed pods which rattle when ripe.
Here I’m sticking to Rhinanthus minor: there is a similar larger flower, Greater Yellow-rattle Rhinanthus angustifolius, but it is rare in Britain.
Yellow rattle – Rhinanthus minor
This wildflower has upright flower stems to 50cm with yellow flowers. Each flower is about 1cm long with two tiny purple teeth on the top lip. The leaves are glossy dark green and serrated. I saw it recently in long grass at Kirkby Moor nature reserve in amongst orchids, where I’m pretty confident it had been sowed.
Although Yellow rattle is a native British wildflower, it is commonly sown in the UK to improve wildflower meadows. Grass competes with the flowers, so reducing the grass vigour by introducing a parasitic species helps the flowers to thrive.
Rhinanthus minor has various subspecies which I don’t feel qualified to comment on!