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Miltonia and Miltoniopsis used to all be considered Miltonia. Then someone figured out that although they are in the same Alliance (Oncidium), they are not the same Genus.
Some differences: Miltonia have two leaves per bulb and the bulbs are more rounded. The flower is a Miltonia flower.
Miltoniopsis has one leaf per bulb and the bulbs are more flattened. The flower looks like a pansy.
Although most people call both Miltonia and Miltoniopsis by the name Milonia, its important to know which one you have because the care is completely different.
Daisy