The Blake's Hitch is an ascending knot, or friction hitch commonly used by arborists and tree climbers in general. Although it is a stable, unslipping knot (in contrast to the arborist's "Tautline hitch", it is often recommended to be completed with a stopper knot (pictured here, a Fig.8) in the end--which becomes very important if the hitch is mistied with the end on the wrong side!) . It is used for both ascending and descending. If the hitch doesn't hold so well on account of slickness of the hitched-to line, an extra turn should be made around the line, with the end still tucked only under two; if the hitching line has trouble gripping because it is relatively stiff, an extra turn should be made and the end tucked under it (which gives more thickness for the stiff rope to compress).
The first known presentation of this knot was made by Heinz Prohaska in an Austrian guides periodical in 1981; in 1990, he presented it in a caver's journal, Nylon Highway. Separately, Jason Blake discovered the knot for himself and presented it to the arborist world in a letter to Arbor Age in 1994; as it was enthusiasticly adopted by arborists, it has become known as "Blake's Hitch".
In practice it helps to insert your thumb under the first two turns. This facilitates threading the end later.
Note: After passing the tail round the standing end. the tail then passes back behind the standing end.