This compositional drawing for the Lock series of paintings is a finished study in which Constable worked out the change from his earlier vertical representations of the subject to the horizontal format of his Diploma picture, A boat passing a lock 1826  . The extension of the sheet reveals the way Constable explored the format and composition when transforming the scene from upright to horizontal format.

Constable depicted Flatford Lock in the foreground, with Dedham Church on the horizon in the centre, and with heavy rain clouds in the sky (which he did not include in the earlier drawing of 1823  ,  or his vertical paintings of the Lock). Here Constable has deleted the barge in the lock chamber waiting to pass downstream (as it appeared in the earlier drawing and the vertical paintings) – but has not yet added the boat with a sail waiting to pass upstream (as in the horizontal paintings). He has depictedthe lock keeper or boatman on the near side of the lock, with one arm raised (his pose is slightly different in all the paintings, and he is on the far side of the lock in the 1823 drawing). The lintel on the posts at the approach to the lock is shown in a different position in the drawing of 1823.The trees to the right are similar to those in the horizontal paintings. This drawing plays an important role in the process of working out the compositional ideas that distinguish the vertical and horizontal versions of the subject.

Reg Gadney has observed that Constable rendered the overcast atmosphere with consummate skill, demonstrating the way he created a pictorial analogy between the textural qualities of his media and the texture of his subject. In this instance, he allowed the wash to run down the surface of the paper to suggest an imminent cloudburst (R. Gadney, John Constable R.A., 1776–1937: A catalogue of drawings and watercolours ...  in the Fitzwilliam Museum, Cambridge, London: Arts Council of Great Britain, 1976, p. 98).

As with Landscape with trees and deer, after Claude July 1825  , in this drawing Constable used a similar manner of handling pen and ink  to that of Claude.