Martin’s 19th century guitars were smaller than they are today and were made for gut strings — guitars strong enough to handle steel strings would not appear until 1928. The original sizes (biggest to smallest) were 1, 2, 2 1/2, and 3. Sizes 000, 00, 0, 4, and 5 were added between 1854 and 1902, and in 1929 this numbering system was scrapped for the one we know today.

The OM, or Orchestra Model, was introduced that year; it is the same size as a 000. The D or Dreadnought (named for the large battleships of the day) appeared in 1916. It was bigger than a 000 and was originally produced exclusively for the Ditson company. But by 1931, Martin was selling its own D-size guitars. It became one of the company’s most popular guitars.

Other sizes in the line include the M (an 0000), the 7 (approximately 7/8ths of the size of a D), and the J (for Jumbo; it’s the same size as a M but as deep as a D), but all of these guitars were introduced on or after 1977, which makes them of little interest to serious collectors.

Style designations have remained consistent since the 1850s, although new designations added in the mid-1980s have made the designation system more arbitrary than it used to be. A 15 is considered a basic model, a 16 a bit less so, with increasing levels of detailing and finishing in models numbered 17, 18, 21, 28, 35, 42, and 45. Thus a Martin D-18, considered a workhorse by many players, is a big, boomy Dreadnought with enough features to make it feel a little bit special, but not so many that you’d be reluctant to pull it out for a couple of songs with friends around a campfire.

Some of the most collectible Martin flat tops include the pre-war Dreadnoughts from the 1930s, which many of the world’s best musicians consider the Stradivariuses of guitars. That said, just about any of the 12- or 14-fret steel-string models from the mid-1920s until the mid-1940s will bring a good price.

Also worth keeping an eye out for are guitars made from the post-war years through the end of the 1960s, when Brazilian rosewood was replaced by Indian rosewood. Indian rosewood is handsome enough, but it lacks the visual richness of Brazilian. More importantly to musicians, guitars made from Indian rosewood don’t seem to sound as good as those crafted from Brazilian stock.

If your ear or eye is not as discerning as a professional musician’s, Martin has made it simple for you to tell which is which: Vintage Martin guitars from 1898 on are easy to date because each instrument is stamped with an individual serial number.

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