First, a big thanks to Rykoshet for a great answer. :)

Originally Posted by: adam_henthorne... in the "more 12 bar blues in A" section of fundamentals 2 i still cant get my head round the time sig it says 4/4 yet i seem to be playing an 8th and a 16th note for each quarter note?

This is "swing time rhythm". Essentially you are slightly delaying the second 8th note of every beat a little bit. How much? Precisely by the amount of an 8th note triplet; thereby forming a tuplet!

Start with even or "straight" 8ths:

1 and - 2 and - 3 and - 4 and -

Insert another 8th note to make 8th note triplets. This results in 3 notes in the space of one beat, instead of just 2 notes:

1 and uh - 2 and uh - 3 and uh - 4 and uh -

Finally, keep the rhythm spacing, but don't play the middle 8th note triplet. This results in the wobbly, slightly off kilter "flat-tire" sound of "swing time". Every "and" is silent:

1 (and) uh - 2 (and) uh - 3 (and) uh - 4 (and) uh -

I did a complete tutorial specifically on this concept here:

http://www.guitartricks.com/tutorial.php?input=542