Double Gummed Stamps

What are Double Gummed stamps? I have seen stamps from the 1890's that have been referred to as being double gummed NH? does this effect the value of these stamps?

I believe it refers to a process where stamps are made to be easy to soak off envelopes. Current US self adhesives are single gummed which makes them almost impossible to soak off.

Self-adhesive stamps that have an extra layer of the type of gum that had been used [before] for non-self-adhesivbe stamps. This was done to make it easier [at all possible] to soak the stamps off cover. Practised among other for Dutch stamps. The Belgian self-adhgesives had this type of double gumming before 2009. Since then the change-over to FSC paper [environmentally friendly] the practise of double gumming was abondoned and since then you cannot [maybe when using chemicals] soak off the stamps without completely destroying them!