[1] It is composed of three articular surfaces: The plantar calcaneonavicular ligament also called the spring ligament forms the whole floor of the talus as it extends inferior to the talus.

It attaches to the anterior aspect of sustentaculum tali inserting into the plantar surface of navicular.

By beginning from sustentaculum tali it covers the plantar surfaces of the middle and anterior articulations between the calcaneus and talus and by attaching to the navicular it covers the articulation between the talus and navicular.

That is a reason why the medial longitudinal arch of the foot is a bit higher than the lateral longitudinal arch of the foot as this ligament is a main part of it.

This article incorporates text in the public domain from page 353 of the 20th edition of Gray's Anatomy (1918)