Abstract This article investigates the etymology of four Latin lexemes starting with /arm-/: arma, armus, armilla and armenta. It examines whether they are men- or mo-derivatives of the root commonly reconstructed as *h₂er- ‘to join’. The combination of an in-depth analysis of 1) the use of armenta in Latin, and ἀραρίσκω and ἁρμόζω in Greek, and 2) similar stems in other IE languages, particularly Vedic īrmá-, Latvian ir̃mi et al., OCS ramo and jarьmъ et al., results in the conclusion that two stems should be differentiated. Armus and the other IE words for ‘shoulder; arm’ point to a second laryngeal and go back to a mo-stem ‘joining, (shoulder) joint’, originally an adjective. Its substantivation process went along with a change in accentuation and ablaut. The middle laryngeal would be the result of a contamination with *pĺ̥h₂-meh₂. The other Latin words and OCS jarьmъ et al. go back to a men-stem ‘the attachment’. The armenta were originally ‘the ones belonging to the attachment (a yoke)’ > ‘the plough animals’. Lastly, it is stated that if ἅρμα was a direct men-derivative of the PIE root, the wheel should be interpreted as ‘the attachment (to the chariot frame)’ rather than ‘the thing joined together’.