Over the last few years, houses across Britain have gradually lost their back walls.
More or less every kitchen extension seems to feature a glass wall at the back that folds back to invisibility, so that the room merges seamlessly with the outdoors.
They aren't the best option for every house - for a start, if they face onto a north-facing garden then you may find it's rarely warm enough to open them all the way. They're also pretty expensive.
Having said that, though, bi-fold doors are by far the best option for maximising a wonderful view - and on a warm summer's evening there's nothing to compare with the indoor/outdoor vibe they provide.
Here are seven homes with bi-fold doors to inspire you.
Shoreditch, London E1
Part of a converted iron works, this three bedroom penthouse apartment features exposed brickwork, beams and Gothic style windows throughout. On the upper floor, there's a large kitchen/living room with a lovely vaulted glass ceiling. Bi-fold doors to one side of the dining area lead out onto a south-facing, decked terrace with a hot tub, sky shower and steps up to a platform with panoramic views across the city. It costs £3.75 million from agent Hamptons International.
This amazing house, designed by renowned architect Tom Wright, is practically all glass - the cinema room is of course an exception and bi-fold doors open up the dining area to the terrace. Some of the ceilings are five metres high, and there are 180-degree views across the countryside. It has four ensuite bedrooms and a huge open living area. Agent Hamptons is asking £3 million.
Part of the Bryant & May match factory development, this two-bed top-floor flat has lovely views of the City. And you don't even need to go onto the terrace to see them, as the kitchen opens right up with a run of bi-fold doors. If you can bear to go downstairs, ther's a shared swimming pool and gym. It costs £525 through Oliver Jaques.
The entire back wall of this house - all thirty feet of it - opens up to a terrace with seating and hot tub. Originally built in 1936, the four-bed chalet home has been extended and refurbished and is now a very modern home. If you fancy a glass of wine while you survey the view, you could always have a word with the owners of the vineyard behind. It costs £1 million from Romans.
Muswell Hill, London N10
This huge double-fronted home has three sets of bi-fold doors in the original arched windows, opening onto a stunning garden with Koi carp pools. It's perfect for entertaining all the guests you can fit in the five bedrooms. There are plenty of period features, including cornicing and parquet floors, and a wood burning stove keeps it cosy in winter. It'll cost you £2.895 million from Hamptons International.
Outdoor swimming pools can be cold; indoor ones can leave you feeling shut in. But the designers of this large house have found a solution, with the pool featuring a glass roof and bi-fold doors to the garden. There are seven bedrooms and half a dozen receptions, with a staff cottage and other outbuildings in the two acres of land. It costs £2.95 million from agent Spencers.
This conversion of a late nineteenth-century magistrates court is now a three-bedroom home, with two bathrooms, a steam room and a large open-plan kitchen. The stunning master bedroom, accessed by a spiral staircase, has wide bi-fold doors opening onto one of two terraces. It's all beautifully stylish, with wooden floors and beautiful stone basins in the master bathroom. It costs £1.3 million through agent Urban Spaces.