A system of usually strong two-suited overcalls devised by Pierre Ghestem of France:
Over 1♦ /1♥/1♠:
2NT shows the lowest two unbid suits.
3♣ shows the highest two unbid suits.
Cue bid shows the other two suits.
Over 1♣:
2NT shows the red suits.
2♦ shows the majors.
3♣ shows the other suits (diamonds and spades).
The modification after an opening 1♣ bid came about to allow an overcall of 2♣ to be natural because, in France, a 1♣ opening is often on a three-card suit.
Another modification of the Ghestem convention uses the cue bid after a minor suit opening to show the extreme suits (spades and the other minor) and the jump cue bid (eg 1♣- 3♣) to show the majors.
When both members of the partnership remember which suits the Ghestem bid shows, the convention works well. However, Ghestem seems to spark more memory lapses than almost any other convention.