One of the reoccurring Hitchcock motifs is the use of the Triangle. Not only as a plot point, but as a visual as well. For example, in both The Lodger and Blackmail, there seems to be one central female character and two potential male love interests creating a love triangle. This is a very common theme told through several of Hitchcock’s films including Mr. and Mrs. Smith, Notorious, Rope, Dial M for Murder, Strangers on a Train, and several other works.
Daisy is involved in a love triangle with Joe and the Lodger.
Alice is involved in a love triangle with Frank and the artist, Mr. Crewe.
Hitchcock used quite a bit of imagery involving triangles between the two films as well.