Tunings ....

Hi All. My first post here. I have just started to play mountain dulcimer - which I'm just loving - so there's plenty I'm still figuring out. One thing I can't figure out relates to how people describe different tunings. CGC, DAA, DAd. 

What is meant by the use of the lower case 'd'. And why not a lower case 'c' on CGC or lower case 'a' on DAA? On the DAd .... Bass string tuned to D, Middle string to A, Melody strings to? D? Then why use small case 'd'. My assumption is this is communicating something that I'm not getting. 

Any thoughts or pointers would help me on in my understanding and be much appreciated.