Coffin built to unusual specifications

‘It took longer, being out of the ordinary.’
On the table, under a half-lit chandelier, the coffin was lying. Holmes turned up the gas and raised the lid. Deep down in the recesses of the coffin lay an emaciated figure.

“‘It should be there before now. It took longer, being out of the ordinary.’ It was the coffin of which she spoke. It had been out of the ordinary. That could only mean that it had been made to some special measurement. But why? Why? Then in an instant I remembered the deep sides, and the little wasted figure at the bottom. Why so large a coffin for so small a body? To leave room for another body. Both would be buried under the one certificate.” (Sherlock Holmes)