Not sure where you read that in Designing With Type but there are 12 points in a Pica and 6 Picas in an inch. Exact measures have changed over time but this is the current standard.

It's possible you're confusing points (typographic) with pixels (digital) or dots (halftone printing) which are not the same. Both pixels and dots are a variable measure and can contain any number per standard measure (e.g. 300 ppi or 1200 dpi).

Hello Ken,
Thanks for your reply and for correcting my mistake yeah 1 pica makes 12point not 15.
thanks for the correction and yeah i am getting confused by it also i would love some great explanation from you guys, what does it help me to know these measures for a line and what is it used for.
Thanks

Picas and points are the north American system. I think there is a different French/European system using Didot points and ciceros. I don't know the rules on that.

Points were developed in the late 19th century, and at the time metric was largely unknown in the Americas. Points were chosen to provide a means of expressing type measurements. Up to that time, type was measured arbitrarily with measurements called agate, pearl, primer, and many more. I think Pica was one of those type sizes and the point worked out to be 1/12 of this.

Until 1970-80 there was a point that was 72.27 to the inch. Computer programmers didn't like that, and they rounded it to 72 to the inch, which makes it much easier for you to remember. The changeover was hell on typesetters though, as we had to compensate for the difference, which builds up to a third of a line on a full A4 page.

Now, if you really want to be confused, look up the dimensions for the en and em.

Actually, a point was always ¹⁄₇₂″; it's the size of the inch that varied! Can't recall what standard Fournier used, but Didot used an inch ¹⁄₁₂ the size of the French legal foot, with his point ¹⁄₇₂ of that. And of course the Adobe PostScript point is ¹⁄₇₂ of the current U.S. inch.

—Joel

Ok guys thanks alot for your reply :)

Points and picas are artifacts of an archaic technology, like cubits and furlongs. "Point size" refers not to any measurable dimension of the letters themselves, but to the pieces of metal the letters would be sitting on, if they were sitting on pieces of metal, which they're not.
Look at this diagram. At the lower right it shows why two alphabets that differ visibly in size can both be called 24-point type.

> it’s the size of the inch that varied!

Really? I did not know that. The US and UK inches have been the same size for the past 500 years. What nations have had different inches?