In the 1970s, large volumes of low cost guitars were being imported to America from the Far East. In order to compete, the CF Martin guitar company, which had always produced high-end acoustics, introduced the Sigma range as a more affordable alternative. The steel and nylon-strung guitars used cheaper woods and materials to keep costs down.
Like the Martin originals on which they are based, the model names of Sigma guitars were intended to provide information about the style of construction and where the model appeared in the range. The "D" in "DM-18" stands for "Dreadnought," meaning the body is a large, round-bottomed steel strung acoustic commonly used in country and folk music. The "M" refers to the mahogany fingerboard used on the neck.
There is some debate about the woods used in the body construction of Sigma guitars, due to a misprint in the original 70s catalog. However, CF Martin customer services now states that all of the Sigma models had solid tops with laminate backs and sides. This configuration is common among guitars at the higher end of budget ranges. The laminate back and sides keep costs down without sacrificing too much in the way of tone. The solid top ensures that resonance is preserved.
The number after the initials denotes the guitar's position in the model range. The Sigma dreadnoughts ranged from 18 up to the top-of-the-range 45. Again, the numbers used reflect those from the Martin range. The DM-18 was the entry-level guitar among those Sigma dreadnoughts with mahogany fingerboards.
Hardware & Dimensions
The Sigma models were all based on Martin's legendary dreadnoughts. The fitted hardware and dimensions were the same as the Martin originals. According to the remnants of the Sigma website, the dreadnoughts had scalloped X-bracing inside the body, chrome tuners and black bridge pins. In keeping with tradition, all of the Sigma dreadnoughts used a scale length of 25.4 inches over its 14 fret necks. According to the Martin website, this is a unique company invention.