Page 99 - Geoffrey Budworth "The Pocket Guide to Outdoor Knots"

Eh, what grief for the poor wee Danny.
He intended a Reef but it turned out a Granny.

You can undo a reef knot by pulling on one working end and breaking the knot into a lark’s head.
It is then easy to slide one part off the other (figure 5).

For extra security, half-hitch each working end to its adjacent standing part (figure 6).