The Flippin–Lodge (FL) angle,  is the latter-derived of two angles that fully define the geometry of "attack" (approach via collision) of a nucleophile on a trigonal unsaturated center of an electrophilic molecule (the second being the Bürgi–Dunitz angle, , see below). Theory and application of these angles falls into the area of synthetic, and of physical organic chemistry (in the specializations of chemical structure and reaction mechanism), in the latter, within a sub-specialty called structure correlation. Studies of   and  can be theoretical, based on calculations, or experimental (either quantitative, based on X-ray crystallography, or inferred and semiquantitative, rationalizing results of particular chemical reactions), or a combination of these.
Nucleophiles in this addition reaction may range from single atoms (hydride, chloride), to polar organic functional groups (amines, alcohols), to complex systems (nucleophilic enolates with chiral catalysts, amino acid side chains in enzyme active sites; see below). Planar electrophiles include aldehydes and ketones, carboxylic acid-derivatives such as esters, and amides, and the carbon-carbon double bonds of particular alkenes (olefins). In the example of nucleophilic attack at a carbonyl,  is a measure of the "offset" of the nucleophiles approach to the electrophile, toward one or the other of the two substituents attached to the carbonyl carbon. The relative values of angles for pairs of reactions can be inferred and semiquantitative, based on rationalizations of the products of the reactions; alternatively, as noted in the figure,  values may be formally derived from crystallographic coordinates by geometric calculations, or graphically, e.g., after projection of Nu onto the carbonyl plane and measuring the angle supplementary to  LNu-C-O (where Nu is the projected atom). This often overlooked angle of the nucleophiles trajectory was named the Flippin-Lodge angle by Clayton H. Heathcock after his contributing collaborators Lee A. Flippin and Eric P. Lodge. The second angle defining the geometry, the more well known Bürgi–Dunitz angle, , describes the Nu-C-O bond angle and was named after crystallographers Hans-Beat Bürgi and Jack D. Dunitz, its first senior investigators (see that related article).
The Flippin–Lodge angle has been abbreviated variously by the symbols φ, ψ, θ, and  or ; the latter pair to closely associate the Flippin–Lodge angle with its sister angle, the Bürgi–Dunitz, which was originally abbreviated as  by its discoverers/formulators (e.g., see Bürgi et al., 1974.). The symbols  and  are used here, respectively, to refer to the Flippin-Lodge and Bürgi-Dunitz concepts and measured values.