Line-cheeked Spinetail is an arboreal member of the family Furnariidae. It is a fairly nondescript resident along the west slope of the Andes and in intermontane valleys of southern Ecuador and northern Peru, where it occupies humid and semi-humid montane scrub and forest. Line-cheeked Spinetail currently is divided into two subspecies based on plumage and size differences between the northern and southern extremes. Yet, between these extremes, spinetails are intermediate in plumage, size, and vocalizations, representing points on a cline rather than diagnosable populations. Moreover, examination of series of specimens from Peru indicates that there is no clear phenotypic or geographic division between Line-cheeked Spinetail and its southerly sister species, Baron's Spinetail (Cranioleuca baroni). These two taxa are best treated as one, following Schulenberg et al. (2007). In sum, the Line-cheeked/Baron's spinetail complex represents a long cline over which body mass increases over two-fold and plumage differences are as great as many interspecific differences within the genus; a truly intriguing biogeographic and phylogenetic puzzle.