Your best friend. Your partner in crime. Your other half.

Not everyone is lucky enough to stumble upon their soulmate of a friend in high school, or even college -- but those of us that do know the drill. What starts as a promising friendship quickly turns into a relationship that lasts the test of time, whether it began in 6th grade at an assembly or during freshman year when you were assigned the perfect roomie.

Before you know it, this person is something more to you than just a friend, or even a best friend. They come home with you during holidays, share your ups and your downs, and know exactly how to push your buttons (and how to avoid it). They become your brother, your sister, your sometimes mom -- and you wouldn't have it any other way.

The official signs that your best friend isn't just a friend anymore -- they're family.

There's no need to knock

You've been at each other's sides so long, knocking has gone out the window. While they would obviously never think of ringing the doorbell on the way inside (because they probably already live with you), they may not even pause before throwing open the bathroom door, no matter your state of cleanliness.

"Oh, you were trying to shower? Well just keep talking while I read you this text so you can parse it for sub-meaning."

You get each other birthday presents...and Christmas gifts...and Valentine's Day treats

When it comes to your friendship, presents aren't just for birthdays. 21 runs were complete with drunk Barbie cakes and tiaras, but those are just the fundamentals. You can also find presents for them under your Christmas tree and wrapped in pink bows for Valentine's Day. Who says boxes of chocolate and pink champagne has to be reserved for your significant other?

You've gotten them arrested, questioned by authority...or at least kicked out of class

You get in trouble together. We've all heard it before, but they really are the person that you would expect to be sitting next to you in running mascara and broken heels if you ever landed yourself in the slammer. Whether you've sprinted away from a busted kegger together, or got her kicked out of 8th grade science class, you've seen your share of adrenaline.

They communicate with your parents -- and often

Your parents consider them one of their kids, and vice versa. This is either the result of a billion middle school sleepovers, or because your bestie is included in every single one of the college stories that you actually tell your parents. Your mom may get he or she a Christmas gift every year, and she gets just as worked up when they talk about moving after college as she does when you talk about it.

You're not afraid to get mad at them

You spend so much time together, sometimes you may get on each other's nerves. This is few and far between, but you're grateful that you don't have to be a constant ray of sunshine around them when you're not feeling it. They're good at reading you, and know when you're actually mad at them versus when you're just mad at the world.

You have a LOT of dirt on them..and vice versa

You can name every boy that they've ever smooched, in alphabetical order. They know your weird fascination with Michael Jackson. Basically even if you wanted to end the relationship, you couldn't...because they could take. you. down.

You have a million inside jokes, and they may date back to 2006

Sometimes you realize mid-laugh that you and your other half have awkwardly been giggling about some joke from three years ago while sitting at a table full of confused people. This is a constant struggle, as nearly anything can jog a past memory with them that you can't ignore. When people ask what is so funny, it's hard to explain that you once got matching sweatshirts that said "Chickenhead" on the back because of that one time she dubbed your 13-year-old boyfriend one when he wouldn't kiss you for the first time.

See? See what I mean!?

When anything amazing happens, they’re the first person you call

You got a scholarship, an interview for a killer job, or you just found the PERFECT dress for your date on Friday. If it makes you happy or excited, you call them. They are always at the top of your text log, because this happens often.

And..when anything not-so-amazing happens, they’re the first person you call

A poor grade on a test you busted your ass studying for, a fight with your significant other, when your parents announced their divorce. Much like you wanting to share good news, you've dialed them with tears in your eyes more often than you'd like to admit. You'll never be able to thank them enough when a death in the family rocked you to your core and you cried on their lap for hours.

You have, had, or want to get a pet together

Getting a dog together at some point has seemed like a REALLY good idea at the time. Never mind that you have zero experience in training animals and aren't prepared to bring a child, I mean dog, into the relationship. It may never work out how you want it to, but MAN wouldn't you be one happy, little family.

You can speak in code when necessary

A side glance here, a seat shift there. Whether you're in the middle of a lecture and something the professor said sounded dirty, or listening to an acquaintance prattle on about how she's doing another juice cleanse, they always know what you're trying to say.

If you no longer live together, there are pictures of them EVERYWHERE

Just like you put up awkward school pictures of your cousins, your best-friend-turned-family pops up all over your apartment. A duck face in the kitchen, that one music festival in the entry-way. It's no surprise that your parents have the other one's senior picture up in the cabinet for all of eternity. You have so many memories together, it's impossible not to smile when you catch a glimpse of yourselves beer bonging on a bad day.

You are protective over them

In high school, my best friend used to say, "I don't ever need to get mad about anything, because you get mad enough for the both of us." And she was damn right. Your partner-in-crime won't be put down on your watch. You develop a sibling mentality, and you have no problem with calling out anybody who you feel is doing them wrong. This includes a lab professor that made them cry, as well as that one dumb kid who told them they were fat in 10th grade. AW HELL NO. No regrets, because you know that they would do the same for you.

If you go without seeing them regularly, a part of you is missing.

The worst thing that can happen -- at basically any age. You were petrified when one of you chose a different university than the other, or you absolutely lost it after 10 years of being attached at the hip was dissolved by a grad school across the country. They are your soul sister, your other half, your partner in crime....and then they are gone.

But you know that the bond you share will never fizzle out. The time you've spent together, whether years or months, will always be a highlight of your youth. It's difficult when you go from leaning so heavily on someone to only seeing their face through Skype, but you'll get through it.

And if you're still blissfully together? Cherish it. Photograph it. Never stop laughing. Because you will never find such a bond that you have found with that one friend, who turned into your family.