Observations of these cool, dense sheets of plasma up in the corona have been summarised in Section 1.8 (see Figures 1.33 to 1.37). Here the main aim is to discuss the magnetic properties of large, long-lived quiescent prominences (Priest 1989; Tandberg-Hanssen 1995; Mackay et al. 2010). In contrast, the important aspects of radiative transfer and spectroscopy are treated by Labrosse et al. (2010). Active-region prominences are much shorter-lived and may perhaps form in a different manner: when they erupt, they normally do so violently, showing up as a spray and giving rise to a two-ribbon solar flare (Chapter 12). Quiescent prominences are much larger and have weaker magnetic fields; they erupt more gently and do not usually produce a solar flare, but the basic eruptive magnetic process may well be the same.

This chapter begins with a summary of observational properties and theoretical questions (Sec. 11.1) and then treats the basic magnetic structure of prominences as current sheets within flux tubes (Sec. 11.2). The global nature of filament channel formation is discussed (Sec. 11.3), followed by the three-dimensional (3D) structure of barbs (or feet) (Sec. 11.4) and threads (Sec. 11.5), which are still a mystery, and the formation of thermal structure (Sec. 11.6). Since most observed flow speeds are much slower than the Alfvén speed, the field is roughly magnetohydrostatic (or even force-free when β ≪ 1).