As soon as they enter the Courtauld Gallery's new summer show, visitors are likely to notice a 16th Century painting that dominates an end wall.
Highly-detailed painted children share the canvas with sketchily-drawn adults.
The painting - seen above - is Perino del Vaga's Holy Family with Saint John the Baptist (1528-37), a work that gives a glimpse of an artistic process interrupted.
Even in this state it has an extraordinary impact, but why would art lovers want to pay to see incomplete works like this?
"Unfinished art can tell us a lot about the materials and techniques and process that artists go through in creating any work," explains Dr Karen Serres, the curator of Unfinished… Works from The Courtauld Gallery.
"It lets us peer over the artist's shoulder mid-work - which we seldom have the opportunity to do.
"At a more aesthetic level unfinished works have a quality and appeal all of their own. We can imagine the possibilities of what they would look like if they were finished, but at the same time they have a ghostly quality that is also very beautiful."
The Vaga work was given to the Courtauld Institute - a leading centre for the study of art history - as a "perfect teaching tool" for students.
"It's very rare," says Dr Serres, "to have a Renaissance work like this with such a contrast between a highly-finished figures and those at a drawn stage."
All of the pieces in the summer show are drawn from the Courtauld's permanent collection and considered "unfinished". They include works by Cezanne, Degas, Manet, Monet and Rembrandt.
Dr Serres, who conceived the exhibition and believes it to be the first of its kind, hopes it will be a "revelation" for visitors.
Another painting from the 16th Century is Venus in a Landscape (c 1520) by Palma Vecchio. The reclining woman is beautifully complete while the sky and rocky mount in the background are unfilled.
After the artist's death a signature was added to the painting by someone who wanted to pass it off as finished.
Death is the principle reason that art is left unfinished, says Dr Serres. "When an artist dies, especially when they have gained some fame, the contents of their studio are dispersed.
"In the case of Degas, his fame was such that even if they were not completed, they were still sold as worthy examples."
Another reason is artistic block. Dr Serres cites Monet's Vase of Flowers (1881-2) which he "struggled with for 40 years".
"An artist might embark on a large scale work but can't crack it. We might think of them as masterpieces but in the artist's eyes they don't know here to go with it and put the canvas aside."
Looking around the gallery, she adds: "Sometimes I think this is not what the artist would want us to see of their work. This is not the face they would want to present. In a way we are looking through the keyhole at what we're not meant to be seeing."
Bare canvas
Some of the works in the exhibition are more ambiguous. The areas of bare canvas in the sky of Paul Cezanne's Route Tournante (Turning Road) have led to academic debate about whether it is finished.
Dr Serres: "Towards the end of his life, Cezanne made a number of these paintings leaving these huge expanses of canvas visible. Some have tried to say he was developing eyesight issues - but it's difficult to think that Cezanne was leaving a whole series in an unfinished state."
So when is a piece of art really finished? Dr Serres says some 20th Century artists find making that decision "incredibly difficult".
"Picasso, for example, considered that to finish a work was to kill it. He said he finished his works very reluctantly. Even contemporary artists sometimes ask visitors to their studio if they think the work is finished or not.
"I think it's still a struggle for artists, but signature is the tell-tale sign."
'The news broke my heart'
By coincidence, in the same week that the Courtauld show opened, an unfinished portrait of Oscar Pistorius went on show for the first time in north London's Zebra One Gallery.
Commissioned by South Africa's Ministry of Sport, it was the last of three painted by artist Natalie Holland, who abandoned the work when she heard of the athlete's arrest for shooting dead his girlfriend Reeva Steenkamp on Valentine's Day 2013.
"The news of Oscar's arrest broke my heart," says Holland. "I couldn't go back to the painting because I just felt so bent out of shape. I didn't want to sell the portrait at the time, even though it would have made most money then - I just wanted nothing to do with it."
It was only recently that Holland decided that enough time had passed to make the portrait public.
Why did she leave it incomplete? "It simply wasn't the same story anymore. When you start painting someone who's a hero, someone that has achieved extraordinary things - being born without legs and running the Olympics, it's quite an achievement.
"But we've ended up with a story whereby the 'hero' goes ahead and kills someone… so it is not the same.
"The painting looks different to me now."
Unfinished… Works from The Courtauld Gallery runs until 20 September 2015. The portrait of Oscar Pistorius is on display now at the Zebra One Gallery in Hampstead.
