This is the low approach.[1] It is absolutely dominant among approach moves to the 4-4 stone. Based on the games recorded in the GoGoD CD, it appears in actual play about 50 times more frequently than the high approach move at a, for example.
The most frequent replies to this approach move in professional play have been (in their order of frequency in GoGoD CD):
Also:
4505 matches (1071/3434), B: 49.7%, W: 50.2%
Ba: 1945 (1936), B51.0% - W49.0%
Bb: 919 (902), B50.7% - W49.3%
Bc: 653 (630), B47.9% - W51.8%
Bd: 551 (521), B46.8% - W53.2%
Be: 98 (97), B49.0% - W51.0%
Bf: 85 (78), B42.4% - W57.6%
Bg: 75 (69), B57.3% - W41.3%
Bh: 35 (27), B54.3% - W45.7%
Bi: 23 (23), B43.5% - W56.5%
Bj: 20 (19), B40.0% - W60.0%
Black may also play tenuki here. That allows White to play a second approach on the other side, called a double kakari. A double kakari of that kind is more severe on Black than in the many variations where Black plays a pincer here and White replies with a double kakari. -- Charles Matthews
Question about diagonal attachment response moved to kosumitsuke joseki.