From the exhibition Many Things Placed Here and There: The Dorothy and Herbert Vogel Collection at the Yale University Art Gallery:

Sol LeWitt created his first wall drawing—a temporary drawing of lines executed directly on a wall and made following a specific set of instructions—for a group ex­hibition in 1968. When the exhibition ended, the work was painted over. The idea for the work, however, remained and could be realized in another location. LeWitt conceived of and first realized Wall Drawing #91 in 1971; it was realized anew for this exhibition by following his instructions: “A six-inch (15 cm) grid covering the wall. Within each square, not straight lines from side to side, using red, yellow and blue pencils. Each square contains at least one line of each color.” The Vogels acquired a number of LeWitt’s wall drawings, one of which was executed by Dorothy and is still on a wall in the Vogels’ apartment today.