As you might know the peptide bond is planar. Therefore it can only be found in cis or in trans. In the cis conformation the C(alpha) atoms point both towards the same direction whereas in trans they point in different directions. This will affect how the R groups pack. A peptide bond is the trans conformation reduces the "steric hindrance" or "strain" because the R groups point in different directions.
Cis bonds are occasionally found especially when proline or glycine (R = H atom therefore not much strain due to tight packing) residues are involved or in areas such as loops and turns. Because cis bonds tend to pack the R groups closer together and destabilize the structure they are often found in areas important for the structure of the protein.
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