A basic move type, in the diagram. A play directly next to another stone of the same color, without touching other stones, to create a line of at least two stones. The second stone "moves ahead" and is solidly connected, hence the English term solid extension.
The nobi advances your stones and creates more liberties for them. You can get an idea of the uses of this move by considering some of the paraphrases for it, such as "stick your head out" and "move out". Often, it is a response to , since if is omitted White will pound Black on the head (hane) at the same point.
Obviously, a nobi is more solid than alternatives to moving ahead such as jumping to the circled point, since by definition it cannot be cut; at the same time, it is a slower way of doing so.
Many players first learn about nobi from the common star-point joseki here, called tsuke-nobi. is the nobi in question.
Narrower and broader definitions
Experts differ on the precise range of meanings of solid extension. Some argue that it is a purely geometrical definition and any stone played next to another of the same color qualifies. Few of even these experts, however, would insist on calling a move forming an empty triangle a nobi just because it was immediately next to another stone. Another group claims that such a broad definition is meaningless, and that solid extension should have the same semantic profile as the Japanese nobi, which is apparently narrower and presumably useful due to its distinguished lexical history. The Japanese meaning of nobi appears to be related to the idea of "moving out" or "moving ahead" or "moving along". To this, the first group responds that move types should not be named based on invisible mental intent.
Consider the following cases. According to the narrow definition of solid extension, aligning it with the Japanese nobi, they are not solid extensions. According to the broader definition, they would be specific types of solid extensions.
- If White has a stone at the circled point: butsukari or bump (TO CONFIRM: butsukari=tsukiatari, according to bump)
- If there are no White stones touching whatsoever: narabi (or possibly iron pillar)
- If Black already has a stone at the circled point: pole connection
- If the move is relatively near the edge and towards the edge: sagari
- If the move is relatively near the edge and towards the center: tachi
- If Black has stones to the right that he is connecting back to: hiki
- If White already had a stone at a: oshi
and of course the list goes on.
Although many people don't think of the relationship this way, nobi can be defined as "a play at a point which, if played by the other player, would be considered a hane (usually; depending on surrounding stones, it could also be an atari, etc.).
Joseki Vol. 1 Fundamentals, p. 149 calls a nobi a "thick extension" and defines it: A thick extension creates thick shape by adding a stone to a string so that the stone has three liberties, the string has at least four liberties and the extension is made from an exterior part of the string's group.
Stretch is less frequently used, possibly an attempt to translate the Japanese nobi, or to create a one-word English term without the potential confusion with extension, which is used to describe fuseki moves such as two-space extension or three space extension.