Compounding is one of the few sequential word formation processes found across sign languages. We explore familiar properties of compounds in established sign languages like American Sign Language, as well as a modality-specific type of simultaneous compounding, in which each hand contributes a separate morpheme. Sign languages also offer the opportunity to observe the way in which compounds first arise in a language, since as a group they are quite young, and some sign languages have emerged very recently. Our study of compounds in a language that came into being only about 75 years ago (Al-Sayyid Bedouin Sign Language) provides insight into the way in which compounds arise and acquire structure. We find in our data a relationship between conventionalization and grammaticalization of compounds: as particular forms become conventionalized in the community, both morphological and phonological structures begin to emerge.