One difference between platanthera bifolia and the similar platanthera chlorantha (greater butterfly orchid), is the arrangement of the two pollen sacs; parallel rather than V-shaped. It a also a generally smaller plant, with more delicate flowers.
Lesser butterfly orchid
Main flower color:
Mostly in southern and western regions of the UK; largely absent from central and eastern areas
Up to 40 cm
Grassland, moorland, open woods, bogs and fens
Two lateral sepals, angled slightly downwards, and a cupped upper sepal, enclosing the two upper petals. The lower petal is narrow, projecting downwards, similar in length to the lateral sepals (around 10 mm), and extending back to form a long (up to 30 mm), narrow spur. Between the upper petals are a pair of pollen sacs, which are parallel rather than V-shaped. All sepals and petals are white or very pale green
Narrow, alternate stem leaves and a pair of much larger, broad, ovate leaves at the base; up to 8 cm long