couple of something—
1. is used of a small approximate number of smth.:
- Sometimes he would go to one of the call-houses and spend a couple of hours with one of the women.
2. is used to indicate two items that do not belong together as a set:
- They had ensconced themselves comfortably in a couple of chairs in the billiard room.
pair of something—is usually applied to items that occur in groups of two:
- The constable took out a pair of bicycle clips, which he adjusted round his ankles.