Constable was repeatedly occupied with the motif of the Lock during the 1820s – it could be regarded as his favourite subject. At the Royal Academy in 1824 he exhibited the fifth in his series of six large canvases of Stour Valley scenes, a painting with a vertical format he subsequently called The lock (Carmen Thyssen-Bornemisza Collection on loan to the Museo Thyssen-Bornemisza, Madrid), depicting the entrance to Flatford Lock viewed from the same spot as this drawing. Constable made at least two other upright versions of this subject in 1824 (Philadelphia Museum and Art Gallery, and private collection). He then converted the vertical composition into the horizontal format of A boat passing a lock1826   .

This is Constable’s earliest known drawing of the composition for the Lock series of paintings. It was made during a visit to Flatford in the spring of 1823. On 21 February 1823 he had told Fisher that he had ‘put a large upright landscape in hand’, which he hoped to have ready for that year’s Academy (Beckett VI, p. 112) – that is, before making this drawing he was already working on a vertical Lock painting.  The drawing resembles the vertical paintings in that the barge is in the lock chamber waiting to pass into the lower level of the River Stour and travel downstream; and a horse is in the fields on the left. The image does not extend to the right to show Flatford Bridge, as in the horizontal paintings. And unlike any of the paintings of the subject, in this drawing Constable depicted the lock keeper on the far side of the lock.

Leslie Parris and Ian Fleming-Williams have pointed out that in this drawing Constable recorded repairs to the lock made in 1820, with  a new angle brace fitted to the near side post, which he did not include in any of the subsequent paintings (Tate 1991, pp. 455–56). The paintings show the entrance in a dilapidated state, with some of the timbers rotting and the bank overgrown with plants. This alsosuggests that Constable had begun working on the painting(s) before his visit in April 1823  (and viewing the lock in a state of repair); or that he preferred to depict the lock in its more ‘ruinous state’.