Pin 1 simultaneously enters three slots, a, d and b, milled in links 2, 3 and 4, respectively. When link 2 is displaced, motion is transmitted by pin 1 to link 4. Link 4 moves as long as pin 1 is in the horizontal portion of slot d of fixed link 3. As soon as pin 1 enters the upper or lower vertical portion of slot d, link 4 stops. Link 2 continues to move until pin 1 reaches its extreme position in slot a. When link 2 is moved in the reverse direction, link 4 begins to move when pin 1 is lifted by slot a to the horizontal portion of slot d. The length l₂ of slot a depends upon the distance l₁ between the vertical portions of slot d.
$2179$WL,D$