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Verizon, AT&T, T-Mobile and Sprint all offer "unlimited" data plans that suggest you can use as much high-speed data as you want each month. But the truth is more complicated. In reality, if you use too much data in a month — even if you pay for one of these plans — your carrier might drastically slow your connection down. If you go over your plan's cap, you won't have to pay overages, but your data might be so slow that you won't be able to do very much online other than load mobile websites and check e-mail. So, while your plan might be technically unlimited in the sense you can use as much data as you want, you're still effectively locked into a data cap because of the slower speeds your carrier might impose after you go over. Recently, the major carriers have added tiers to their unlimited data plans with larger caps of high-speed data. It's confusing. I took a look through the so-called unlimited plans offered by the big four U.S. carriers to find out what you actually get when you pay for "unlimited data." They all have some perks that attempt to make the plans more enticing, like free TV, international data or a subscription to Netflix, depending on the carrier. But I want to show you, specifically, what the term "unlimited data" actually gets you in each case. If you're interested in the other perks, click the link to each plan below. Here's what I learned.

Verizon

Verizon has three unlimited plans priced at $75, $85 and $95 per month, respectively: "Go Unlimited," "Beyond Unlimited" and "Above Unlimited." The pricing for each decreases as you add more lines to the account. What you need to know: Verizon's Beyond Unlimited plan is capped at 22GB of high-speed 4G LTE data per month, while the Above Unlimited is curbed at 75GB. If you hit these ceilings, Verizon reserves the right to slow your data speeds. The entry-level Go Unlimited is unique. Unlike the more expensive plans, there's no guarantee of a certain about of high speed unlimited data you can use without seeing throttling. Verizon explained to me that "data may be temporarily slower than other traffic during times of congestion" at any point during your usage. Also, while all three plans offer a hotspot feature that lets you use your phone to provide internet to a laptop on-the-go, data for that sort of usage is also limited depending on the plan. Streaming video is limited to 480p (that's DVD quality) on Go Unlimited and 720p on Above Unlimited and Beyond Unlimited. Even if your smartphone has a 1080p or sharper display, the video won't appear as good as it technically could. I like that Verizon spells out all of this right on the page where you pick your plans, instead of burying it elsewhere.

AT&T

AT&T offers two unlimited plans, and, like other carriers, the pricing changes depending on how many lines per account. For a single line, AT&T offers "Unlimited & More" and "Unlimited & More Premium," which cost $70 and $80 per month, respectively. What you need to know: AT&T caps both unlimited plans at 22GB of data per billing cycle, after which it may slow your speeds down. The difference between the two plans comes down to video quality and hotspot data. Like other carriers, AT&T caps video at 720p by default, though the the "More Premium" plan offers 1080p streaming and 15GB for mobile hotspots. Your hotspot data gets slowed down to a measly 128kbps after you hit that cap, which is barely enough to check your email or send an iMessage.

Sprint

Sprint has two unlimited plans: Unlimited Basic, which starts at $60 per month for one line, and Unlimited Premium, which starts at $70 per month for one line. What you need to know: Both of Sprint's "unlimited" plans are very limited. Unlimited Basic only supports 480p video, streaming music at 500kbps, playing video games at 2Mbps and 500MB of LTE hotspot data. Just to put those speeds in perspective, a modern 4G LTE connection should get you about 50-100Mbps, so you're getting 2 percent of the speed you pay for when you're playing video games, and less for streaming music. Unlimited Premium ups the ante a little, but it's still extremely slow. You can stream in 1080p but music streaming speeds are limited at 1.5Mbps, gaming streaming is limited to 8Mbps. You also get 15GB of LTE hotspot. Sprint will throttle your speeds if you exceed more than 50GB of data in a month but, as you can see, your speeds are throttled from the get-go for most things anyway.

T-Mobile

T-Mobile One starts at $70 per month, though the price drops as you add more lines. T-Mobile advertises "unlimited talk, text and data." You can upgrade to T-Mobile One Plus for $10 per more a month, which includes a few more features. What you need to know: T-Mobile One includes 50GB of high-speed 4G LTE data. Once you go above that, as with other carriers, T-Mobile can drop this to slower speeds. Also, while T-Mobile includes hotspot tethering, it's capped at 3G speeds which means you're going to be sitting around for a while if you need to download a big file. Video streaming is capped at 480p, too. You can pay T-Mobile more if you want to avoid some of these boundaries. T-Mobile One Plus adds support for streaming in HD, offers 10GB of 4G LTE hotspot data per month (3G after that), faster data speeds if you travel abroad and free Wi-Fi on flights equipped with Gogo.

So what do you get?