Once upon a time the placket theory was the theory that the Burgundian gown was worn directly over the chemise with the neckline being filled in by a placket. These days they placket theory is a little different. Sometimes when you look at paintings of this style of dress the hem of the under dress is a different colour to the fabric seen at the neckline. The placket theory these days describes a Placket of a different fabric pinned to the underdress, so that the underdress looks one colour at the neckline and another at the hem when lifted. This also gives the look of the smooth fronted underdress while enabling the wearer to wear a front laced kirtle under the gown.
Here one can clearly see a placket under the gown.
1460, Rogier Van der Weyden, Portrait of a Lady. If one looks at the neckline of this dress, the partlet appears to cover part of a kirtle. The layering here could be kirtle, partlet, placket, gown.
1495: Portrait of a Woman of the Hofer Family, Artist unknown. Here you can see the red overlaps th black of the kirtle. The red fabric could be a placket pinned over the black kirtle.
1430-35, Rogier van der Weyden, The Descent from the Cross (detail). Front laced Kirtles are very popular in 15th century art, however one rarley sees front lacing under the gown, suggesting that they would pin a placket over their front laced kirtle before donning a gown.