An additional note to add to Ken's explanation - I always tab for both D-A-dd and D-A-AA for our dulcimer club tunes ,but I always show tab for all three strings, even though all of our tunes can be played on just the melody string or with chords, as desired. This is the first time I've seen tab like this, with only the top two strings shown for each. I can't say I get around very much, but in my experience this isn't a very common tab style.

In a comment on my I Just Got A Dulcimer, Now What? blog, Evy Bush asked:

I understand how to read all the tablature except the ones that are set up like this one where you can choose between DAD or DAA.

I just can't make sense of which strings are which because any way I look at it they don't fall in order. Can anyone explain this for me? Thanks! I, sure it is something very simple that I am somehow missing.

Here's a link to an example: http://www.everythingdulcimer.com/tab/amazingddul.pdf "

The subject comes up fairly often so I thought I'd answer publicly, so others can benefit.

The link is to Amazing Grace posted by RLWalker:

The top two lines, marked AD are for those who are tuned to DAd, and represent what you, the player must fret on the Ad strings of the DAd tuning.

The bottom two lines: DA, are for those tuned in DAA, and show you what to fret on the A melody string of the DAA tuning.

This particular illustration is the perfect example of how to "convert" DAd tab to DAA and vice versa. If you have DAd tab and want to play in DAA, simply add 3 to the d melody line numbers and play the new numbers on the A melody line in DAA. If you run into a 6+ fret notation in DAd, play that as a fret 9 in DAA.

updated by @ken-hulme: 06/09/16 08:21:50PM