You may live under the same roof, be raising children together and have an intimate connection with one another's dirty sock collection. But do you really know your other half as well as you think you do?
"It's so important that we get to know our partners well; so that we're not only aware of the obvious personality traits, but also the ones which are buried deep, which may only be exposed during times of stress or vulnerability," explains Stefan Walters, a psychological therapist at Harley Therapy who specialises in couples counselling and family therapy.
"We all act differently under great pressure, and frequently resort to 'adaptive behaviours'; coping mechanisms which we've developed earlier in life, to help us deal with painful feelings and events."
As a relationship continues, undoubtedly some of these 'adaptive behaviours' start to be revealed, and instead of fretting that you are showing off a side of yourself that is vulnerable or unappealing, according to Walters, these emotions - and a partner who is attuned to them - can be helpful in getting you through a difficult moment.
"Many of the strongest couples are the ones who can recognise each other's adaptive behaviours early on, as warning signs that something isn't right, and then soothe each other until the danger has passed," he says.
We asked seven couples in relationships how well they really know each other - here's what they told us.
Do you really know each other as well as you think you do?
M: We know enough to trust each other unconditionally. We communicate in a way that helps us get through anything.
W: Yes... but I always want to know more.
M: I can usually correctly guess what she will think about important subjects based on her values which are very strong and clear. We can complete each other's sentences most of the time.
W: Yes, I think we do know each other as much as we think we do and we think alike in so many ways. We can easily anticipate each other’s reactions towards any event. It is sometimes scary when we can complete each other’s sentences or think of the same joke at the same time. We have a very similar sense of humour.
M: I believe so, even if it sounds a bit presumptuous.
W: How can I answer this? I think I know him - time will tell if I do or don't...
M: Yes, but I enjoy learning more about her each day.
W: I think so. We've been together a long time and I think I have a good grasp of who he is and what makes him tick.
M: Yes, we do. We have so many things in common - a mutual enjoyment analysing friendships and relationships and each other's hobbies and foibles. I love sending her news articles or recommending books and TV shows she'll like and vice versa.
W: Sometimes I feel I can tell exactly what he’s thinking, because I recognise his expression or mood from so many times before (and I know the reason for his grumpiness or whatever he’s feeling). I also know that the same things make us happy. But I love that he can still surprise me and that I’m always discovering something new about him.
M: I would hope so; I often feel that we share the same thought or joke without having to express it in words and she understands all my foibles. In many ways I can anticipate how my wife will react to a particular position. This is no great display of emotional intelligence on my part, it is more a sense of understanding how I would feel in the same place, because we often respond the same way. We do not so much as finish each other sentences, but rather, start them.
W: I'd like to think so. We talk about everything, communicate well but I love still finding out new things about him. For example; he took four years to tell me he had seen Michael Jackson live. The Bad Tour. This is amazing! This should have come up in maximum the first five conversations.
M: Unfortunately, yes.
W: I think so. We have known each other since we were 19 (we are now 32) so, in a way, we have grown up together. We also have small children and when you have small children, your social life is slim to none - so we spend all of our time together.