In her long and useful paper on the biology of the land snail Helix lutescens, Elżbieta Koralewska-Batura describes the initial stages of mating as follows.
...two of them assume a vertical position and remain in further contact through their mouths, tentacles and raised anterior parts of their feet. The snails are supported on posterior parts of their feet and on their shells, most often stuck in concavities in dense herbaceous vegetation. Almost all the surface of their soles adheres closely, and the gonopores* widen.All Helix species seem to go thru the same pre-mating stages of behavior. I had a chance to photograph a couple of mating Helix in October 2008 in Turkey.
The snails' vertical positions indicate that they had just started their courtship. When I took this picture, they were slightly disturbed by my intrusion into their affair and hence withdrawn their tentacles. Unfortunately, I was with a group of people and could not afford to stop and watch the rest of the process at leisure. I hope I will see and photograph more next time.
Koralewska-Batura, E. 1999. Helix lutescens Rossmässler, 1837 (Gastropoda: Pulmonata: Helicidae) Its structure, biology and ecology. Folia Malacologica 7:197-240. Abstract