Reference at the time of writing this article: ICC Playing Conditions – applicable as of 2024.
“Field Restriction Circle” is an integral part of Limited Over Cricket (both ODIs and T20s) introduced to dictate or control the number of fielders that can be present at certain parts of the field of play at different phases of the game. This is not a feature that can be found in Test Cricket.
This circle is very commonly referred to as the “30 Yards Circle” (and in Women’s Cricket it is 25.15 Yards). Though the word “Circle” is popularly used whenever it is referred to, in reality it is not a “Circle”. And it is not an “Oval” too.
The shape is more of two opposite-facing semicircles at each end of the pitch with straight lines joining them together.
The semicircles are drawn at each end of the pitch from the center stump, which marks the end of the pitch (the bowling crease). The semicircle drawn is extended only to this point and not extended till the line of the popping crease. These two halves are later joined together by a straight line.
Another interesting point about these markings is that, irrespective of the size of the boundary (shorter, longer, irregular) this region will not change and has to be maintained uniformly.