The diachronic process of assimilation is ubiquitous in the scholarly literature on sound change. However, the literature on assimilatory processes in language change has focused almost exclusively on spoken languages in the oral- aural modality. To address the gap in understanding of assimilatory processes in other modalities, here we consider the role of language modality in assimilatory processes in signed language; specifically, we ask how modality-influenced differences in the phonological structure of the spoken word versus the sign may condition the types of assimilatory diachronic changes observed in the two modalities. We argue that analysis of diachronic assimilatory processes in the gestural-visual modality requires both the traditional notion of temporal/sequential adjacency—i.e., the articulation of a sequence of gestures over time—as well as consideration of adjacency of different articulators in space at a given point in time, in other words, spatial/simultaneous adjacency. Using examples from early 20th century video recordings of American Sign Language (ASL; cf. Supalla 2001, http://hsldb.georgetown.edu/), along with preliminary data from comparative basic vocabulary lists for 13 signed languages currently being gathered by the authors for a larger study of signed language change, the paper will discuss (i) types of temporal/sequential adjacency assimilation in signed languages and (ii) assimilatory changes spurred by spatial adjacency. Temporal adjacency (i.e., sequential adjacency) is the de facto domain of essentially all work on assimilation to date: in this type of adjacency, at least one feature differs across the structural units of a word or a sign at times t1 and t2. Meticulous typologies of assimilation (e.g., Pavlik 2009) have scrutinized various temporal/sequential relationships that can hold between two temporally adjacent segments. The centrality of sequential adjacency assimilation in spoken language change in general is consistent with the observation that the oral-aural modality is a “serial channel” that favors sequential structure (Pinker & Bloom 1990: 713; cf. Brentari’s 2002 discussion of the auditory system’s greater facility in “horizontal processing tasks” based on Bregman 1990). The gestural-visual modality, in contrast, has been considered a “simultaneous channel”, one that favors simultaneous rather than sequential phonological and morphological structure (Brentari 2002, Sandler 2017). These modality-influenced differences in phonological structure affect the frequency with which sublexical units within words and signs occur sequentially, which in turn implies that sequential adjacency assimilation is likely to be less prominent in the history of signed versus spoken languages. In signs, the simultaneous co-occurrence of sign parameters, or formational components, has led some scholars to argue that signs are typically monosegmental (Channon 2002, Van der Hulst & Van der Kooij 2021). If most signs indeed consist of a single segment, then the type of sequential adjacency across segments that triggers many diachronic assimilatory changes in spoken languages may not occur frequently in signed languages. See, for example, Frishberg’s (1975) typology of diachronic change in ASL, in which she adduces only examples from morphologically complex signs, mainly historical compounds, to illustrate her principle of assimilation and fluidity. However, many monomorphemic signs—not just compounds—also exhibit sequential adjacency, in particular signs with multiple phonetic values within a single phonological parameter. For example, in video and print sources from the early 20th century (e.g., Long 1918), the ASL sign UNDERSTAND included two phonetic values in the location parameter: contact at the center of the forehead with the tip of the flexed index finger, followed by a location with no contact at a slight distance from the forehead with full extension of the index finger. In the putative contemporary ASL reflex, there is no contact at the forehead, and the change in handshape (a change in aperture, cf. Brentari 2002: 47) occurs at a slight distance from the forehead. That is, the initial location has evidently assimilated to the second location. The transition from one phonetic location to another created the type of sequential adjacency that allowed for a diachronic assimilatory change— one that occurred in a monomorphemic (and putatively monosegmental) sign. Thus, it is clear that, in defined contexts where sequential adjacency is relevant, we see evidence of assimilatory processes in signed languages. Whereas sequential adjacency is logically more pervasive in spoken versus signed languages, spatial adjacency assimilation is, arguably, particularly prominent in—if not exclusive to—the gestural-visual modality. This type of spatial adjacency is visible in the case of some two-handed signs. As noted by Frishberg (1975), in many historical signs in which two distinct handshapes were articulated simultaneously, the nondominant handshape tended, over time, to shift to match the dominant handshape; the ASL signs LIQUOR, INSTITUTE, and HARD all exemplify this assimilatory change: they all had different handshapes in the dominant and nondominant hands in early recordings of ASL, but have matching handshapes in contemporary ASL. Similarly, in contemporary French-Belgian Sign Language—a language that, like ASL, putatively belongs to the French sign family—both dominant and nondominant handshapes are similar in the sign HARD. Although Frishberg characterized this type of diachronic handshape change in ASL as symmetry, we argue that it represents a process of assimilation motivated by the spatial adjacency of two distinct articulators. We conclude that spatially adjacent assimilation is prominent in signed languages precisely because the gestural-visual modality is a strongly “simultaneous channel” of communication, one which makes use of multiple articulators and a larger articulatory space, allowing assimilation processes motivated by spatial adjacency to take center stage in the phonological evolution of signed languages.