An operating mechanism is provided for a screen assembly having an elongate endless pull cord hanging from the screen assembly in a loop. The operating mechanism has a housing having an opening at an upper end thereof. At least a lower portion of the pull cord can pass into the housing to be enclosed therein. A guide means for guiding passage of the pull cord through the housing. The guide means has a locking means adapted to selectively restrain first or second sides of a loop defined by the pull cord. The guide means includes a cord-retaining member engaging a lower end of the looped pull cord. The cord-retaining member is moveable within the housing between vertically spaced first and second positions, the second position being above the first position, the cord-retaining member being biased towards the first position by biasing means. The housing may be displaced downwardly with respect to the pull cord by upward movement of the cord-retaining member between the positions against the force of the biasing means.
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