LOS ANGELES - In recent weeks, Apple, Google and Amazon.com have each launched the missing puzzle piece in their wireless mobile music systems.
Apple enabled storage and delivery of your songs over the Internet through iTunes Match. Google started selling music digitally. Amazon shipped an electronic-books device, called the Kindle Fire, that does much more than books.
With those additions, each system now lets you buy songs, store them on faraway computers called the cloud and retrieve them wirelessly on devices connected to the Internet.
But which system do you want to live with? It's a choice you can't make lightly because these companies don't play nice with each other. Once you've adopted one, it's hard to switch.
If this were the Music Cloud Wars, then Apple's iTunes Match would be winning - but not by much.
Here's a quick primer, along with a few ways to get in and around their digital barriers.
There's a good chance you are familiar with iTunes. The software is on millions of computers, and many of you have iPods, iPhones or iPads that let you consume content bought through the iTunes online store.
ITunes Match is a $25-a-year service on top of that. It sees everything you have in iTunes and matches it to copies Apple already has stored in the cloud. Songs not already there will be uploaded from your computer to a personal locker in the cloud.
It's alone among the three to let you download songs to iPhones and iPads wirelessly. That means a full copy of the song is stored for listening anytime, rather than streamed on demand over wireless networks, which can be spotty. There's nothing more annoying than having your songs stop and start as your connection flutters.
You can have up to 25,000 songs on the service, plus an unlimited number bought through iTunes - great for those with large music collections. Of course, most of you won't fit 25,000 songs on your device, so streaming is an option for songs you haven't downloaded yet.
If there's a tune you want to listen to offline, just tap an icon. It takes only a few seconds, and you can start listening before it's done.
One major caveat: You need an Apple device to use this, and specifically a newer one with Apple's iOS 5 mobile software. You're out of luck if you have a phone running Google's Android system, for instance.
Using Google's free Music Manager program, you upload music you own into Google's cloud. Unlike Apple, Google doesn't have songs preloaded, so this can take hours or days.
Google Music works best with an Android phone or tablet computer. You simply download the Google Music app to your device. Voila, your songs will be available for streaming. You can save songs for offline playback by "pinning" them with a digital push pin icon.
The service stores up to 20,000 songs, not including those bought through a companion music store run by Google. That's not as many as iTunes Match, but it's free.
I like Google's music store because it offers plenty of bargains. I found Coldplay's latest album, Mylo Xyloto, for $5 - half the price on iTunes. Google plans to release lots of free music, too.
I also like that if you buy from Google's music store, you can share the songs with friends on its Google Plus social network. They get one full listen for free - that's something not available anywhere else.
One downside: Google's store isn't as extensive as Apple's or Amazon's. For instance, it's missing songs from Warner Music Group, which accounts for about 20 percent of music sold in the U.S.
Google Music also isn't a great option for users of Apple devices.
Google found a way to make the system work on iPhones and iPads through Apple's Safari Web browser. It has a surprising app-like feel because of the way menus respond to touch. But Apple device users won't be able to store songs for offline use.
There's also a trick for Apple users to take advantage of music deals: Download the songs onto a computer, put the music in iTunes and upload the songs into Apple's cloud through iTunes Match. It's not pretty, but it works.
Amazon Cloud Drive.
The new Kindle Fire completed Amazon's music system, though it's not required. It works fine on Android devices through the Amazon MP3 app.
Released in March, Amazon's cloud storage system is free for up to 5 gigabytes of storage - roughly 1,250 songs. If you bought Lady Gaga's latest album, "Born This Way," in a 99-cent promotion in May, you'll have 20 GB of space - good for about 5,000 songs.
Amazon's uploader works about the same as Google's. It could take hours or days to get your songs into the cloud. But once there, you can stream or download songs to the Kindle Fire or to Android devices.
Like Google, Amazon sells songs and albums at a discount to iTunes, and its long-running music store has a selection comparable to iTunes.
Amazon has also found a way to make its system work on Apple devices, using Safari as well, but that workaround is clunkier than Google's and doesn't support downloads either.
One other downside to Amazon's service is that you'll likely have to pay for cloud storage, as you do with iTunes Match.
Having 5 GB of storage for free is kind of meaningless because most mobile devices have that already. The Kindle Fire comes with 8 GB on board. For a limited time, you can get 20 GB of storage for $20 a year - and most music files won't count against the total.
Although there are things to like about Google's and Amazon's systems, they both favor streaming, which isn't how I want to listen to music when I'm not at a computer.
Apple's iTunes Match is fundamentally more oriented to work with downloading in mind, and it meshes well with your existing song library, either on your device or on your computer.
The iTunes store is also set up better - showing what's new and popular, and acting as a barometer of popular culture. Google promotes what's free and Amazon emphasizes its bargains, but those picks aren't always what I'm looking for.
Ultimately it's great to have cloud services out there. It has helped me organize my music collection and reconnected me with songs stuck in the recesses of my computer.
In the end, though, these services ought to be as free and easy to access over multiple devices as email is. Instead, they come across as tools to get you to buy this or that device. And we shouldn't be made to pay for a song once and then again when we store it.
Music in the cloud has promise, but it hasn't fully delivered just yet.