An L-shape slotted deck board and hidden fastener system includes a decking board or plank and a hidden-type fastener device. The plank includes two generally L-shaped slots formed on a planar underside or bottom of the plank, which provide the construction member with two underside, inwards-facing shoulders or lips. The fastener is a clip-like member that includes a flat body portion and two “wing” portions on opposite edges of the body. Each wing includes a sidewall portion perpendicular to the body, as well as a lip portion attached to the sidewall and bent down inwards toward the body. The wings are complementary in shape to the slots, such that the lip and sidewall portions of the wing “hook around” the inwards-facing shoulder of the L-shaped slot. The fastener holds adjacent planks together by hooking around the planks' laterally adjacent, oppositely facing slots/shoulders.