skylight?

Don't want to make this too long as to why but do the skylights (velux roof windows) sit ON the rafters or between. I have a truss roof timbers and am told by local loft company they can't be cut, distance between trusses is 16-18".

Thanks

Oh! If they sit ON the rafters I will go for a wider one even thought a truss will be showing. Obvious it would have to be a top hung one. Needed for ventilation in hot weather as conservatory covers the windows.