The axion models originally proposed by Wilczek and by Weinberg chose axion coupling strengths that were so strong that they would have already been detected in prior experiments. It had been thought that the Peccei-Quinn mechanism for solving the strong CP problem required such large couplings. However, it was soon realized that "invisible axions" with much smaller couplings also work. Two such classes of models are known in the literature as KSVZ (Kim – Shifman – Vainshtein – Zakharov) and DFSZ (Dine – Fischler – Srednicki – Zhitnitsky).