In the real world, if you want a top 100 album, you can drop by a strip mall and pick it up at Sam Goody or Tower Records.
If you want the six-LP, clear vinyl boxed set of David Bowie's "Sound & Vision," however, you'd have to search hard for a store that carries it.
Online, the divisions are not as neat, but they do exist.
It doesn't take much time, money or technical skill to open an online music store with an inventory larger than that of any mega-store, with major distributors handling the shipping and returns for most online music retailers. The key difference lies in stocking the online shelves with hard-to-find music titles.
Amazon.com began its life as a bookstore, but it moved into music last year. Almost immediately it became the largest online music retailer, even though competitors such as CDNow have focused on music from their inceptions.
The features Amazon uses in its other categories, such as one-click ordering, sophisticated reviews and articles, and CDs by independent artists, all work in its online music store. Also, because Amazon warehouses its products itself, you can buy a book, a toy and a CD and receive all of them in one package, minimizing shipping costs.
CDNow is the largest online store focusing on music and is a favorite among aficionados. But its shipping charges can be onerous, depending on the number of items ordered.
Sam Goody and Barnesandnoble.com will not accept returns from their online stores at their physical stores, which negates their biggest potential advantage.
Borders stores, however, will accept returns from Borders.com, while Tower stores will do so only in some cases, although a customer service representative could not say which Tower stores would accept them.
Global Music Network, which includes content from British Broadcasting Corp., focuses on classical and jazz and has its own online radio station, along with detailed information about each recording that's playing at any moment.
Tunes.com offers an extensive archive of concert videos available for viewing and a community feature that lets shoppers read what other shoppers recommend.
For those resisting that jump to the digital world, Harvard Square Records Inc.'s LPNow.com sells only vinyl albums, including the much-sought-after "The Body Rules," a 45 rpm from Minnesota's Gov. Jesse "The Body" Ventura by Twin/Tone Records in 1984 that inexplicably failed to get much air time.
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Web sites reviewed: Sam Goody: http://www.samgoody.com; Amazon.com: http://www.amazon.com; CDNOw: http://www.cdnow.com; Barnesandnoble.com: http://www.bn.com; Borders: http://www.borders.com; Tower Records: http://www.towerrecords.com; Global Music Network: http://www.gmn.com; Tunes.com: http://www.tunes.com; Harvard Square Records: http://www.lpnow.com.