Manacles
Blake talks about slavery and imprisonment, both literal and figurative, everywhere. And by all over, we mean all over. In his short works, in his long works, in his unpublished works, in his… well, everything. And what's Blake's most frequent way of talking about this theme? He uses words like "manacles" and "chains." A lot. If you've glanced at "London," you know that it contains that famous phrase "mind-forg'd manacles."
"London," of course, isn't the only one. Many other poems in Songs of Experience use similar words. In "Earth's Answer" there's "chain'd in night" (14) and "Break this heavy chain" (21). In "The Tyger" you've got "What the hammer! What the chain" (13). And in "Little Boy Lost" there's "And bound him in an iron chain" (20). Clearly, then, chains, which are just "manacles," are everywhere in the Songs of Experience. Blake was big on pointing out restrictions, then trying to break folks free with his art.