List Manipulation : {{{a,b},{c,d}},{{e,f},{g,h}}} into {{a,b},{c,d},{e,f},{g,h}}

Catenate@list

where

list = {{{a,b},{c,d}},{{e,f},{g,h}}}

{{a, b}, {c, d}, {e, f}, {g, h}}

Flatten might be something you need:

listNested = {{{a, b}, {c, d}}, {{e, f}, {g, h}}};
Flatten[listNested, {1, 2}]

output:

{{a, b}, {c, d}, {e, f}, {g, h}}

You can also use Join:

Apply[Join] @ {{{a, b}, {c, d}}, {{e, f}, {g, h}}}

{{a, b}, {c, d}, {e, f}, {g, h}}

Alternative/equivalent form: Join @@ {{{a, b}, {c, d}}, {{e, f}, {g, h}}}

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