Typically, the first half of the corners First-half lines are largely ignored by internet bookmakers. They simply take an over/under market line for the whole match (i.e., over/under 9,5 corners) and split it in half, usually deleting one corner (i.e., over/under 4,5 corners). This means that if you were to back each half individually, you would be taking huge risks since there is no way to know which half will have more goals scored in it. However, since the second half of the corners tend to be more intense than the first, many bookmakers will set different odds for each half. So if you were to back both halves equally, you would be taking less of a risk since there is some variation between them. Of course, if you knew which half would have most goals scored in it, then this strategy would not work but since they are equal in popularity, most bookmakers don't show any preference for one half over the other. The reason why the first half of the corners tend to be ignored by bookmakers is because they believe most matches will be close enough that neither team will need too many chances. Thus, they assume that only one or two goals will be scored and so setting odd-numbered corners as unders isn't worth their time.