At Shop It To Me, we believe companies can disrupt a market and have long-term loyalty not just by building a great or insanely fun product, but by building an indispensable product.

Look at some of the services today with the most avid users — Google Search,  Apple’s iPhone (when it first came out), Twitter, Etsy, eBay, Pinterest — all have one thing in common: they have built a product that is indispensable for certain audiences.

What is indispensable?

So, what exactly is an indispensable product? I believe you can divide it into three different components:

1. An indispensable product solves an important or meaningful problem.

Every indispensable product out there solves important problems for its users. VCs often refer to this as as the “aspirin” vs “vitamin” scenario (whenever you have a headache or pain, aspirin is a must-have; vitamins are a nice-to-have).

There’s a reason Google Search is so popular — it is indispensable in two ways. Users of Google search trust it to give them answers to the most important questions. For advertisers, Google SEM and SEO has traditionally been the best place to find customers with active intent to buy their service. Selling your amazing new tax software for businesses? Get to be at the top of the search results for “business tax software” and you’ll have the huge number of highly targeted leads you need to crush your quarterly goals.

2. An indispensable product has no good substitutes.

To gain real traction, an indispensable product not only needs to solve an important need; it must lack good substitutes when it first comes out. Your product won’t be indispensable if users can easily find an alternative.

When the Apple iPhone first came out, there were no other products remotely like it. It was terrible as an actual phone, but it was the only phone out there for consumers that would let you actually search and view real web pages (as opposed to just mobile versions), or see your emails in a visually appealing and simple way.

If you want to build an indispensable product, you need to make your product unique for the customers you are going after — you can’t just be a slightly better version of a popular product and expect people to switch.

3. An indispensable product is ideally something you need on a frequent basis.

The third point is not a true requirement of indispensability, but an important attribute if you want to build a habit and get frequent usage. If people find your product indispensable but only need it once every 5 years (or once at all), you may have a great product, but you won’t be building a habit for when competitors enter the space. On the other hand, if people need your product frequently, you have the ability to train them to be accustomed to your service. This will keep users coming back long after other competitors make similar products (think of the millions of people still using MyYahoo! 10 years later).

A quick test for indispensability

So you think your product has all three of the criteria for an indispensable product. How do you know for sure that it’s indispensable? Here’s one easy test: take it away from your users and see how they react. If people start screaming that the service you provide is gone, there’s a pretty decent chance you have an indispensable product.

Think about the products that are indispensable to you. Smartphones, webmail and Twitter are all indispensable to certain people. Think of how people tense up when their phone goes missing for 15 minutes, or how a reporter would feel if they could not access their Twitter feed and had to wait until news appeared on a website.

From inadvertent tests, we know our Shop It To Me emails can be indispensable. Every once in a while our emails get delayed. When that happens, we often hear about it not only from our data but from our support box —  users email us demanding (occasionally with profanity) to know why their salemail has not yet arrived. And with our new product,  Shop It To Me Threads, we occasionally test the waters of indispensability by asking user-testers how they would feel if we removed certain features. We’ve had a number of features that users say are “really great” that we removed from our system because they didn’t notice when they were gone.

So for all of you working on the next big thing: as you build out your product, and start prioritizing features, figure out what parts are needed to make your product more indispensable. Focus your energies on that. You’ll build a stronger product and have a much larger chance of turning your idea into a wild success.

This post originally appeared on the author’s blog.