At the risk of giving away the secrets of musicians everywhere, there are bound volumes of (sometimes legal) musical scores called "fake books." Rather than denoting every note and rest within a composition, they instead offer a "lead sheet" made up of a single melody line and chord headings. It's then the musician's job to devise an arrangement (read: fake their way through) based on this bare outline. The most well known of these fake books is the Real Book, which is full of jazz standards.
I mention all this to give you some idea where iReal Pro (Mac App Store link) gets its name. (iReal Pro is available in versions for iOS, Android, and the Mac; I discuss the Mac version, which costs $20, here.)
iReal Pro is more than a collection of musical scores (known as "charts" to us hep-cats). It's additionally an auto-accompaniment application rudimentarily similar to PG Music's $129 Band-in-a-Box. The idea is that you select a chart and press Play, and iReal Pro plays a three-instrument backing track--drums, bass, and piano (or guitar), for example. Your job is to play or sing along with this virtual band.
You have the capability to choose from among 35 song styles: 15 jazz, 9 latin, and 12 pop. The instruments and accompaniment patterns you hear are based on the style you choose. For instance, the jazz charts include keyboard (acoustic, electric, or vibraphone), bass, and drums. Pop charts include guitar as one of the instrument choices. You can also change the tune's tempo and key.
The interface is straightforward. Along the left side of the window is a Library pane where you choose from among libraries and playlists of tunes. Just to the right of that is a list of songs in the selected library or playlist. And on the far right are the chords and arrangement for the selected song. By default, the currently playing measure is highlighted in yellow (though you can change the highlight color as well as the color of the background "paper"). When you press the Play button you hear clicks (the count-off) that indicates the song's tempo; the highlight moves through the chart as the song plays, so you know which chords to accompany.
By default, you're supplied with 50 exercise charts. This isn't much to start with, and it certainly doesn't reflect the content of the real Real Book. However, when you first install the app, you're cleverly asked if you'd like to visit the iReal Pro forums. And you should, because it's within those forums that you'll find links to the tunes you really want to play. For example, choose the Jazz forum, and you'll find links to a load of jazz standards. Click one of these links and the charts are automatically added to iReal Pro and gathered in a playlist. No melody lines or lyrics are included, which I suspect is how the app's developers get around any copyright issues--after all, if you could copyright chord progressions, the Blues bin at the local record store would hold exactly one album.
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