The centres of circles forming a coaxal system are **collinear** 2.

A and a are called the "pointers," since they are **collinear** with, or point to, the pole-star.

The **collinear** centres are the three sets of one external and two internal centres, and the three external centres.

- the sum of the moments of the forces about each of the three points which are not **collinear** must be zero.

The origins of these four plane co-ordinate systems may be **collinear** with the axis of the optical system; and the corresponding axes may be parallel.

Three **collinear** stars ?', c and 3 Orionis constitute the "belt of Orion"; of these E, the central star, is of the ist magnitude, 3 of the 2nd, while Orionis is a fine double star, its components having magnitudes 2 and 6; there is also a faint companion of magnitude io.