An opening is a small section
A window reveal is the return from the outer face of a wall to the frame. From an oblique angle it can make the depth of a wall visible. A shallow opening may read as a line; a deep opening can produce a small volume of shade. Neither appearance alone tells you the full wall build-up.
Observe a window from more than one position without looking into private rooms. The edge, head, and sill may use different materials or profiles. Draw a simple section through the sill and label only the parts you can actually see.
Light changes the reading
A reveal that seems dark in one photograph may be brightly lit at another time. Orientation, season, nearby buildings, and the reflectance of the surface all matter. Note the time and sky condition if you plan to compare openings. A photograph without those details is still useful, but less controlled.
The frame position affects the visible depth on either side. Record whether it appears near the outside face, near the inside, or somewhere between. Avoid assigning thermal or waterproofing performance from this observation; the crucial layers may be hidden.
The edge is where systems meet
Look for a sill projection, a drip, a sealant joint, or a flashing edge. Follow the geometry rather than assuming that a visible line has a particular function. A material change can be a finish joint, a movement allowance, or part of a more complex assembly.
Bring the sketch back to a reliable construction reference and compare possible arrangements. Keep the original observation separate from the later interpretation. That simple distinction makes a reference collection much more trustworthy over time.