There are friends who pass in and out of your life, and there are some people who make an indelible mark.

If you're very lucky, you have a person in your life who you see as an extension of yourself, and here are just 9 signs they'll be around for a very, very long time.

How many sound familiar to you?

1. Distance doesn’t breathe animosity

There are some friendships which require your constant attention, and others which survive and thrive regardless of contact and distance.

The sign of a true friendship is the ability to slip back into it… no matter how long you've been apart.

If you have a friend who you can call at 2am despite the fact you haven't chatted in weeks, you're onto a winner.

2. Their win feels like your win, and vice versa

It's hard not to feel envious of other people's triumphs, but this certainly isn't an issue in this friendship.

Because you see this person as an extension of yourself, you feel their joy as deeply as your own.

Major or minor, no one will celebrate their achievements as sincerely as you will. And it works both ways.

3. You’ve name-dropped them more than once

If you've ever experienced a genuine surge of pride over the fact you can call this person your friend, you are on track for a lifelong friendship.

You know how amazing this person is and you can't help but bring them up on the regular….even if no one asked.

If it wasn't mutual, it might even be weird.

4. Mutual understanding plays a huge role

One sign that you're in it for the long haul with this person is the fact that you don't expect more of each other than either can deliver.

You both know you have your strengths and your weaknesses when it comes to friendship, and you're happy to accept them, as long as you're honest about it.

Your importance in their life is never under any question, so when they have to cancel a dinner or turn down an invite, it's no biggie, because chances are they'd rather be with you… but you know… life.

5. Silence is never ever awkward

If you have someone in your life that you'd happily just nap with, you're onto a winner.

Meeting up to sit in silence may sound odd to some people, but they haven't met you guys.

You get a kick out of simply being with them, and if a three-hour hang-out session involves a 90-minute nap, 30 minutes of unbridled snacking and 60 minutes of Selling Sunset, so be it.

6. You hide nothing from them

And we mean nothing.

You have told them things you wouldn't dream of telling anyone else and you have admitted things to them you once struggled to admit to yourself.

And the best part? You know – hand on heart – that you can do so, without judgement.

7. You call each other out when necessary

Most friendships encounter a few hiccups along the way, and yours is no different.

But unlike other friends whom you have to treat with kid gloves, you can tell your friend like it is.

Support comes in various forms and telling your friend they're behaving like a jackass is just one. Let's face it, it's better coming from you than their boss, co-worker or flatmate.

8. You seek, receive and provide reassurance for one another

When it comes to advice or a shoulder to cry, you turn to this friend more than anyone else.

Sometimes it comes down to their own experience of a situation and sometimes it because you simply want them to listen, but either way, when the going gets tough, it's this person whose reassurance you seek.

And even if they can't help on any practical level, you can be sure they'll take your mind off your woes.

9. Their habits become your habits

When you find yourself using their sayings and making their gestures, you know this person is going to be hard to shake off (Not that you'd want to.)

In fact, the only thing you still argue about is tea.

The day you see them opting for a mug over a cup, Supermilk over regular, and no sugar over their stand spoonful, you'll know they've passed into a different realm. You'll have literally become the same person.