The Senate's amendment process evolved over the years and is based on a continued interpretation of past parliamentary practice. Those precedents stipulate the nature of amendment that may be offered at a particular point in time (i.e. first or second degree; perfecting or substitute).
Which amendment tree the Senate is following at a particular point in time is determined by the first amendment offered to the legislation under consideration.
Chart 1: Amendment to Insert Text
Chart 2: Amendment to Strike Text
Chart 3: Amendment to Strike and Insert (Substitute for Section of a Bill)
Chart 4: Amendment to Strike and Insert (Substitute for a Bill)