The Inner Ring Grind is a grind trick in which something (typically the thumb) is placed inside the ring of one half of a yo-yo while spinning, thus grinding on the object. These are not possible with all yo-yos due to sidecaps or lack of an adequate rim weight groove. Inner ring grings became very popular in the early to mid 2000s, prior to the introduction of hubstacks. Many yo-yos were designed with a careful attention to weight distribution, which resulted in an "inner ring" to maximize rim weight. This integral design element became the basis for the popularity of IRGs.