A pet hate of mine is to see double rebated casements in a traditional cottage or barn conversion (see further down)

The above image is how it should be done with the casements (the opening part of the window) flush with the front of the frame.

Note also that there is no timber sill in local windows - the slatestone sill is let into the walling either side so as to prevent rainwater getting in at the ends of the wall opening, it slopes back up and sits under the bottom of the window frame.

The bottom of the frame itself should have an angled rebate to receive the stone sill and the frame is scribed to fit the uneven slate sill (you can see that the wood is thicker on the right than on the left where the timber frame meets the sill).