From Chess Programing Wiki

NIC-Key, a system of classification for chess openings, variations and subvariations by NICBase, New In Chess. Instead of the abstract ECO codes like 'A' to 'E', the openings are given lettercombinations that follow directly from the descriptive opening names, like 'SI" for the Sicilian Defence and 'KI' for the King's Indian Defence. Variations and subvariations are given numbercombinations that enable it to give every sub(sub)variation a unique code. The NIC-Key has been used internationally since 1984 and according to the NIC-site, is still evolving.

Main Keys

AL Alekhine's Defence : 1.e4 Nf6
CK Caro-Kann Defence : 1.e4 c6
FR French Defence : 1.e4 e6
IG Italian Game : 1.e4 e5 2.Nf3 Nc6 3.Bc4
KF King's Fianchetto, : 1.e4 g6
KG King's Gambit : 1.e4 e5 2.f4
KP King's Pawn Openings : 1.e4 e5
PU Pirc Defence : 1.e4 d6
QO Queen's Gambit : 1.d4 d5 2.c4 e6 3.Nc3 Be7
RG Russian Game-Petroff : 1.e4 e5 2.Nf3 Nf6
RL Ruy Lopez-Spanish : 1.e4 e5 2.Nf3 Nc6 3.Bb5
SD Scandinavian Defence : 1.e4 d5
SI Sicilian Defence : 1.e4 c5
SL Slav Defence : 1.d4 d5 2.c4 c6
SO Scotch Opening : 1.e4 e5 2.Nf3 Nc6 3.d4
TD Tarrasch Defence
VG Vienna Game : 1.e4 e5 2.Nc3