The term Jacob's staff is used to refer to several things, also known as cross-staff, a ballastella, a fore-staff, a ballestilla, or a balestilha. In its most basic form, a Jacob's staff is a stick or pole with length markings; most staffs are much more complicated than that, and usually contain a number of measurement and stabilization features. The two most frequent uses are:in astronomy and navigation for a simple device to measure angles, later replaced by the more precise sextants; in surveying for a vertical rod that penetrates or sits on the ground and supports a compass or other instrument.

Adriaan Adriaanszoon, called Metius,, was a Dutch geometer and astronomer born in Alkmaar. The name "Metius" comes from the Dutch word meten ("measuring"), and therefore means something like "measurer" or "surveyor."