"Senior Guides" slowly came into existence as some captains (adult leaders of companies) formed patrols of girls over 16 years old.
Robert Baden-Powell, the founder of Scouting and Guiding, was interested in this development and in June 1917 asked Rose Kerr to take on responsibility for them, outlining to her a plan for them.
In June 1920, Olave Baden-Powell, then the Chief Guide, wrote: Here is the suggested new name: "Ranger".
A Ranger is "one who guards a large tract of land or forest," thus it come to mean one who has the wide outlook, and a sense of responsible protective duties, appropriate to a Senior Guide.
Another definition is "to sail along in a parallel direction," and so we can feel that the Ranger Guides are complementary to the Rover Scouts.
They can pursue any or all of several schemes including The King's Guide and the Duke of Edinburgh's Award.
[1] Members of Rangers often play a wider role in the running of Girlguiding as they take on additional responsibilities.