Is called enveloping of a curves family (C ) a curve CS which accomplish the conditions: for each point which belongs to the CS curve, is possible to find a unique curve from family which contain this point as ordinary point and which have in that point a contact of first order with CS; for each (C ) family's curve is possible to find an ordinary point of this curve which belongs to the CS curve too; nor of the (C ) family's curves don't have a common arc with the CS curve.