stops chain suck

Bolts through the "chainstay bridge" ( i.e. the little tube between the chainstays - see final pic). 

the spare shorter bolt is for bikes with a threaded mudguard hole in the "chainstay bridge"

this is made for triples, but should be able to be positioned for doubles too.

if you have no hole through your chainstay bridge, be prepared to drill one 

it could be made to fit bikes with no "chainstay bridge" with a longer bolt and top plate to sandwich top and bottom of chainstays (much like you'd see on a bike stand)  - but you'd need to modify / add parts yourself for that. 

note: this item will fit most bikes, but not all, please check your bike is compatible before buying.