The whole point of drawing a "red line" for one's opponents is to send a clear signal when enough is enough. Unfortunately, for Israeli Prime Minister Benjamin Netanyahu, the chart he used today for his "red line" speech on Iran's nuclear program was kind of confusing. Here's why.

The casual news reader is probably aware that the basic issue over Iran's nuclear program is the purity of its uranium. Specifically, the focus is on Iran's production of uranium enriched to 20 percent purity, a level that puts it in close reach of weapons-grade uranium. Story after story focuses on that percentage, and U.S. and Israeli officials have repeatedly said that getting Iran to halt its 20 percent uranium enrichment is the "top priority." Oddly, though, that number was missing on Netanyahu's chart. Instead another number loomed large: 90 percent.

Even to some experts, who have reported on the subject for years, it appeared like Netanyahu was drawing the red line at the wildly generous level of 90 percent enriched uranium. Here's seasoned foreign policy reporter Laura Rozen:

Did Netanyahu screw up? No. But he did use a different yardstick for conveying Iran's nuclear capability than is typically read in newspapers. By 90 percent, Netanyahu meant it couldn't allow Iran to complete 90 percent of the work required to reach weapons-grade uranium. He wasn't referring to the specific purity of acceptable uranium enrichment, a point made by Stephen Young, a senior analyst and Washington representative of the Union of Concerned Scientists. As Reuters explains, Netanyahu was really "referring to Iran's enrichment of uranium to 20 percent purity, a level [Iran] says is required for medical isotopes but which also brings it close to bomb-fuel grade." One of the fears is that Iran could accumulate enough low-enriched uranium for a weapon. Just as an example, the Institute for Science and International Security projected last year that Iran would need until late 2012 to accumulate enough 19.75 percent low enriched uranium for a weapon. So this is where the puzzlement lies.

Amusingly, the confusion about the chart has been completely overshadowed by the fact that the cartoon bomb included in Netanyahu's poster resembled a Looney Tunes-style prop, an issue that went on to divide the nation.

Go figure.