The very rare fen violet (Viola persicifolia) knows two varieties in the Netherlands. Both are endangered.
Fen violets have tall erect stems that can be as tal as 10 to 30 cm in the var. persicifolia to only 5 to 15 cm in the var. lacteaeoides
They are easily recognised by narrow leaves and always pale flowers that open white als soon turn into very pale blue.

The var. lacteaeoides could be mistaken for some forms of Viola canina as it is also a small plant but the rounded flower, pale colour and narrow leaves make it recognisable.
Both varieties of Viola persicifolia sometimes grow together with V. canina and a natural hybrid between these two (Viola x ritchliana)excists. This plant is however very rare and dificult to recognise, so I leave it out of this key for now.

Besides the characters mentioned below in the key itself, it is also worth to mention that these two varieties differ in spur length.
The Viola persicifolia var. persicifolia is has a shorter spur which extends the sepal appendages with just one mm.
The the spur of the var. lacteaeoides is larger and extends these appendages with around two mm. while the whole plant is in fact smaller.