joining vertices causes a crease to appear

hi all. this is difficult to explain so i put some photos up . basically im modelling an audi tt and ive been using the snap tool to temporarily join vertices. however ive now neered my completion and want to join the vertices together. when i do this the result is a crease that appears ( i dont know what to call it) as soon as the vertices are joined . why does this happen. Im thinking that i may be doing something wrong ( im a newbie) or is it the application . thanks in advance for your replies.


Try use Ctrl+N to recalculate normals after you join the vertices