Enter the serial number stamped on the neck block, visible through the soundhole. See the year your D-18 was built, its design era, and which of the 25 documented changes between 1934 and 1953 it should have, plus what changed through 1964.
Every confirmed serial number helps map how Martin built these guitars. Fill in what you know, copy the message, and send it to The D-18 Listening Project on Facebook.
How to read the results. By serial means the change is pinned to a known serial number. By year means the change happened during that year but the exact serial isn't known yet, so a guitar from that year could go either way. Check it means Martin built both versions, and only the guitar itself can tell you.
Martin serial numbers run across every model. This lookup assumes the guitar is a D-18. Year boundaries come from Martin's last serial number for each year, and the changes come from Bryan Kimsey's bench documentation and the Did You Know? Facebook series. Changes from 1954 to 1964 come from published sources and have not yet been documented on project guitars.