As someone who has had the same best friend for the past 15 years, I know a thing or two about why she might be, in reality, my soulmate. A true best friend is not just someone you complain to and hang out with. They become a part of you, someone you absolutely could not live without. Here are some signs that your BFF is a soul friend.

1. Mental Synchronization

Not only do you finish each other's sentences, but you're sometimes so in sync that it's scary. Lots of times, my best friend and I accidentally wear the exact same outfit, or just a similar style. Once, a year apart and without knowing it, we bought the same tank top in the same size. We often say the same thing at the same time, and even make the same faces at each other. A lot of our inside jokes just come from us saying weird things at the same time.

2. You're Often Mistaken For Sisters, or Even a Couple

My best friend and I are so close and in sync that people assume we're sisters. When we hang out, people will ask us who is older or who gets away with the most with our parents. We haven't been mistaken for a couple (that I know of, yet), but we do hold hands and cook and clean for each other. We've been asked before if we were twins (and we look nothing alike).

3. Personal Boundaries Don't Matter

I've been to my best friend's house so many times that I can get their extra key and let myself in without telling her. I can go to the fridge and get something to eat without asking. I even know where they keep their vacuum (under the stairs, by the office) and remember that their gas stove used to make an annoying clicking noise before they got it fixed.

Another thing that doesn't exist: personal space. Need something to keep your toes warm? Use your best friend. Need a place to rest your head? Your best friend's shoulder or even lap can work. Lindsey and I even share the same bed when we sleep over, or go to the beach or somewhere else. When you're as close as sisters, personal space doesn't exist.

4. You Used to Hate Their Significant Other

When your best friend is your soulmate, it hurts when they start to seriously date someone else. My best friend felt this way when I started dating my boyfriend, and then I felt that way when she started dating hers. When your best friend has been your person for as long as you can remember, it feels almost like you've been cheated on when they get a significant other. What do you mean, you love that guy? You're mine!

Don't worry, the feeling won't last, unless the significant other in question is horrible. My best friend approves of my boyfriend, and I hers. Although, if either one of us gets hurt, the other will likely respond violently.

5. Non-Verbal Communication

When you spend enough time with someone, your communication transcends finishing each other's sentences. You begin the non-verbal communication that is an important key to the soulmate-level friendship. When someone does something crazy, you two share a look. You don't even need to point out the craziness you just witnessed; your eyes are already shouting about it.

6. You Are, Sometimes Literally, Partners in Crime.

For Christmas last year, I bought one of those cheesy best friend heart necklaces that are broken in half. But instead of "best friends," it says "partners in crime." When you're with your best friend, you tend to get in trouble. Safety in numbers, right? For instance, my best friend and I once pretended to break into her house while her brother was home. Long story short, we scared him pretty badly and almost had the police called on us. When we went to the mall with our parents, we used to run off and hide in clothing racks. A few years ago, when in Charleston for a cousin's wedding reception, we snuck through a mostly empty plantation house turned hotel and explored any and every unlocked door. I don't have any regrets about these things. After all, life is much more fun when you have a partner in crime.

7. You Always Celebrate Each Other's Victories...

...and know when to tell each other to give it up.

My best friend has always been my number one cheerleader (next to my mom, that is). She is the solid foundation of my support system. I'd like to think that I've supported her as much as she has supported me. When one of us succeeds, we both celebrate. In fact, when I was accepted to Wofford, I found out at her house. Needless to say, we celebrated into the wee hours of the night with ice cream and movies.

The other side of being each other's support system is knowing when to tell each other to stop hanging onto what is dead. You pick each other up when you fall, and help each other get back to being on steady ground.

In short, I would be nothing without my best friend.

I love you, Lindsey!