There is a drawing that gets to the root of Maurice Sendak’s ominous sweetness, his work’s potent mixture of childhood idyll and threatening night.

It’s a sketch of a costume for the premiere of Oliver Knussen’s early-1980s operatic adaptation of “Where the Wild Things Are,” the picture book that had made Sendak a publishing sensation two decades earlier. The costume is for one of the maniacally grinning Wild Things, complete with horns and pointy-sharp teeth.

But the drawing is a cross-section. Inside the looming beast is just a child, his little hands and feet strapped into the woolly Wild Thing’s, making the character roar by speaking through a tiny cone.

The boy in the monster, the monster in the boy: This is the reality Sendak, who died at 83 in 2012, wanted us to see, and understand.