The fourth stroke is hooked. However it frequently is illustrated in compound characters （like 督) without a hook but this is dependent on the font. In Skritter, the font that appears in my browser as text in the HTML (to the left of the Flash window) shows the hook, Skritter itself does not.

Where as in Pleco the character lookup shows the hook in a compound but the (Apple) font in Pleco doesn't.

Now, normally it's not a big deal but in this case this is a defining feature which Skritter is actually looking for and will mark you wrong if you hook the stroke.

Yes, I have noticed this as well

I've noticed this too, for this exact character. I wrote in feedback, but the Skritter guys wanted to keep it as is

That is because the 叔 in 督 is an “upper component” (上偏旁/上边旁). The rule for this can be seen here: http://www.edu.tw/files/site_content/M0001/biau/t08.htm?open . The website is for the traditional characters, but this rule also applies to the simplified ones.

Thanks, pts. Very interesting and useful link and information.

I'm curious, could the rules in Taiwan be different from those followed on the mainland? Checking that link from pts, for example, the 匕 (on its own and in 旨 and 匙) has a 横 where I learned a 撇 as the first stroke, and the 余 in 途 still has that 钩 on nciku...

According to 說文解字, the oldest Chinese dictionary, both 旨 and 匙 has a 横. The URLs are http://dict.variants.moe.edu.tw/yitia/sa/sa01774.htm and http://dict.variants.moe.edu.tw/yitia/sa/sa00417.htm respectively. It can be seen that some latter variants use a 撇. Taiwan tends to follow the original shape of the characters in order to preserve the meanings contained in the components. The mainland usually used the variants or simply creates a simplified one. There are countless differences between the traditional characters used in Taiwan and those “traditional” characters used in the mainland and Skritter.

As for the hook rule, actually, most Chinese are not aware of it and write randomly (hook or no hook doesn’t matter). Usually the first encounter with this rule is when one starts learning calligraphy. Originally, it’s purely for aesthetical purposes. It’s synthesized by analyzing the writings of the calligraphers and deduced that this will usually result in some better looking characters. But here comes the computer age and everything has to be standardized, and this rule becomes official in Taiwan.