Triptychs usually had one important biblical scene in the middle with donors or saints either side, but in The Garden of Earthly Delights, all three panels are equally part of the story.
The use of monochrome or what is known as grace for the outer panels of a triptych was quite conventional and is there to set the stage for the explosion of color within.
He had found in Rome an early altar of honey-colored stone and had been dickering in Florence for six months for a triptych of the Siennese school to put over it.