The onset of the folding effect characteristic of highly viscous liqui
d films (plane jets) slowly impinging on a wall is studied. Nonlinear
quasi-one-dimensional equations are derived to describe the how. In th
e linear approximation they reduce to the eigenvalue problem, whose so
lution predicts that instability (the onset of folding) sets in when t
he length of the film exceeds a critical value. The critical folding h
eights and the oscillation frequencies at the onset of instability are
predicted as a function of how parameters. Theoretical results are co
mpared with Cruickshank's (1988) experimental data. Agreement is quite
good only in the range of parameters where the quasi-one-dimensional
approximation is applicable (thin films at the onset of folding).