Pivoting rail having tonneau cover system

A tonneau cover assembly for covering a cargo box of a pick-up truck, the cargo box being bounded by spaced walls, including a forward wall toward a cab of the truck, side walls and a rear wall, said tonneau cover assembly comprising:

• a frame having a plurality of frame rails, including a forward rail, side rails and a rear rail, said frame rails being joined to one another at ends thereof to form said frame in a rectangular shape, said frame for positioning on top surfaces of said spaced walls;
• a cover of a flexible sheet material attached to said frame;
• said forward rail including a base rail and a pivot rail;
• and
• an elongated base rail including a body portion for engaging a top surface of the forward wall, said body portion having a first pivot portion and a first conjunct feature, said first pivot portion having an open, unobstructed forward end portion;
• a pivot rail including a body section, said body section having a second pivot portion and a second conjunct feature;
• said first and second pivot portions being cooperatively engaged and permitting relative rotational movement of said base rail and said pivot rail between a closed position, an open position and a release position;
• said first and second conjunct features being cooperatively engaged in said closed and open positions whereby said pivot rail is prevented from disengaging said base rail in said closed and open positions;
• and
• said first and second conjunct features not being cooperatively engaged in said release position whereby said pivot rail is disengageable from said base rail in said release position, said first and second conjunct features not being cooperatively engaged solely as a result of movement of said base rail and said pivot rail into said release position.