Bob Klein writes:

  

I see a french dovetail the same as a sliding dovetail.  The female side being slightly tapered so as to tighten as the male part is slid into place.

 

pictures may help explain, unfortunately the only ones i could find were on new junk.  As Indrek said the older pieces sometimes had the cross piece under the drawer attached into the side of the case with a sliding dovetail but many were half dovetail.  If you have ever worked on an early wardrobe the mechanism that attached the top and bottom to the sides is a variation of the sliding dovetail.

 

bobklein
pensacolafl