If there is one perennially-true statement about the business world, it’s this: Successful companies are built on solid relationships, including your employees, vendors and — most critically — your customers. The degree to which you build strong, positive relationships correlates directly to the health of your organization.

I see this in my company all the time. The clients we’re most friendly with are the ones that stick with us long-term. Even when we suffer a bump, they remain solidly attached to our brand.

The same goes for my employees. I see better attitudes and output when I’m actively working on maintaining positive relationships. When I get busy and let my relationships falter, problems often follow shortly after.

The reason isn’t complicated. Human beings prefer to associate with the people and by extension the companies they like. If they feel you care about them and are working hard to keep them happy, they’ll forgive the occasional hiccup and establish themselves as ambassadors for your brand. Want referrals? That’s how you get them.

Related: Maximize the Impact of Handwritten Notes With These 6 Tips

Handwritten thank you notes laser focus on relationship building

What do handwritten thank you notes have to do with business relationships? Everything, it turns out, and I’ve built a successful company on the truth of that statement.

Before the internet, email, text messages and even the telephone, handwritten notes were the medium of choice for connecting with others. Cuneiform, the earliest extant written language we have, was born in ancient Mesopotamia, some 5,200 years ago. Our ancient relationship with the written word has forged a deep emotional bond with the medium.

Gratitude shares its own longstanding bond with human cultures. Rules concerning social reciprocity can be found across the globe, and individuals that honor them achieve more rewarding relationships.

Gratitude provides both the recipient and the sender numerous benefits, including increased well-being, a more positive outlook, reduced feelings of loneliness and isolation and closer, more meaningful relationships.

When you take the time to send a client, partner or employee a thoughtful handwritten thank you note, they’ll feel seen and appreciated. They’ll pay your efforts back with loyalty.

Related: Why a Handwritten Thank-You Note Should Be a Staple Email Strategy of Luxury Brands

Loyalty is extremely valuable to your organization

What good is loyalty? It’s your organization’s single most powerful currency. Loyal, repeat customers are worth their weight in gold (almost literally.) Joint analysis from Bain & Company and Harvard Business School found that a slight increase of 5% in customer retention rates could translate to a hefty 25% to 95% boost in profits.

HubSpot‘s research confirms the value of satisfied customers. The venerable platform’s customer acquisition study found that a whopping 90% of consumers are more likely to do additional business with a company that keeps them happy. Additionally, 93% said they give repeat business when offered an excellent customer experience.

Loyal employees are just as valuable. They get to know your customers and clients. They help you build external relations. High employee churn disrupts these valuable relationships, damaging future prospects.

When your employees feel appreciated for their contributions, they devote themselves to your service, and their productivity goes up, overall production increases, your corporate culture improves and your company’s image gets a boost with customers. And all it takes is the occasional handwritten, “thank you for everything you do for us.”

Related: Why Gratitude is the Best Marketing Plan Ever

Handwritten notes offer a real ROI

Handwritten thank you notes aren’t just a nicety.

We know a thing or two about handwritten thank you notes at my company, because we generate more than 1 million of them for our clients each year. We see the dollars and cents of business gratitude every day.

One of our larger customers in the home services sector is an excellent example. They’ve integrated automated gratitude into their customer aftercare process. Each time they provide service, they use Zapier to trigger a warm, handwritten thank you, which arrives in the mail a few days later. When they return for the next scheduled service, often a year later, their technician finds their thank you note stuck to the fridge or sitting on a counter.

Their inexpensive handwritten notes translate to greater brand awareness, neighborly word-of-mouth referrals, and more business. That client and others in the industry regularly report that their automated handwritten gratitude programs increase the long-term value of their clients by more than 300% compared to traditional print and email campaigns.

Create a culture of gratitude and hit your goals

Business gratitude works best when it’s consistent, authentic, and specific. Let’s break that statement down. You need to give thanks whenever it’s warranted, every time it’s warranted. Your gratitude should be heartfelt and authentic, not templatized and insincere. Finally, you need to be specific about what provoked your response so that your recipient fully appreciates the depth of your thanks.

Creating a culture of gratitude is the best way to attain high marks in all three criteria. When you and your employees make a habit of expressing thanks to each other and your customers, consistency won’t be a problem.

This routine also provides practice with honest self-expression, creating a company steeped in the subtleties of authentic gratitude. Finally, consistent appreciation forces you to pay attention to why you’re sending out thanks.

Business relationships matter because companies and individuals are just people, and people crave close personal bonds with others. If you can provide that along with a strong product or service, you’ll have customers for life. Handwritten notes are the gateway to deeper, more meaningful and more profitable customer relationships.