An oscillator is said to be T-nosed if, at one (and only one) point in its evolution stage, a detached T-pentomino forms at an edge of the pattern, specifically in a way that allows it to interact as a finger.
Currently, T-nose oscillators are known for periods 4, 5, 6, 7, and 8.
The pentadecathlon is generally not considered a T-nose as, despite the fact that it does produce an isolated T-tetromino in one of its phases that lends itself to catalysis, the cell at the point of that T is also alive four and five generations afterwards, where a true "T-nosed p15" would have these cells dead for all 14 other generations of the oscillator.[1] Instead, the pentadecathlon is a phi-sparker. The same applies to Tanner's p46.
For the same reason, the second oscillator below[2][3] cannot be called a "T-nosed p8" as a blinker created by it turns the cell at the tip of the T on two extra times.
Oscillators like the pulsar and pulsar quadrant are also not T-nosed as the fingers they produce come from an L-tetromino, rather than a T-pentomino, even though they can interact in much the same way (see boring p24 and uninteresting p24).
Oscillators such as Rich's p16 and Karel's p15 can be said to "act" like T-noses, as they do produce a cluster of cells identical to a T-pentomino in one and only one phase and allow for the same catalysis, however as the T-pentomino is in fact a subset of a larger polyomino they are generally not counted among the T-noses.
There are at least three known p4 t-nose reactions, with the T part remaining the same but the immediate hassling region differing. An Octagon 3|5 can support two reactions simultaneously,[4] with the 3 side being the same as the reaction the conventional T-nosed p4 uses.
Reactions for p5, p6, p7 and p8 are known and all have been stabilised as of August 2022. Other true t-nose reactions of this period and true t-nose reactions of any other period are unknown.
No still life (allowing for a "T-nosed p1") can contain a T-pentomino within itself due to the central cell immediately dying of overpopulation and the tip dying of isolation, and no period-2 T-nose can exist because those two cells cause the oscillator to recede two cells, which cannot be made up in a single generation. It seems very likely that no true t-nose reaction can exist for period 3.