Question
we just bought a new clematis plant and all it said was to trim as of class B. What is class B and when do we trim it?l

Answer
Don't be intimidated by these Clematis classifications.  These simplify your Clematis choices.  If you understand them they will help you grow your Class B Clematis better!

Clematis fall into three Groups, based on ways to prune them.

GROUP A (Group 1) - Clematis that bloom in early Spring on growth from the previous year: C. alpina, C. macropetala, C. armandii and C. Montana, which grows well as far North as USDA Zone 5.  A healthy specimen of any of these becomes very large but should be pruned to keep them attractive.  But they must be pruned immediately after flowering; pruning them toward the end of the year, prior to next year's spring flowering, removes that year's flowers.  That's when you find people writing in asking what's wrong with their Clematis, it never flowers.  The trouble with classifying these is that no one explains this.  Now you know!

GROUP B (Group 2) - These are large flowered hybrids you see in catalog pages of Wayside Gardens and White Flower Farm.  A Group B/Group 2 Clematis blooms on old wood.  You prune this only when you want to remove spent flowers.  There are also two Subgroups, Group 2a and Group 2b.  A 2a Clematis bloom in Spring and sometimes repeats in Fall.  A 2b Clematis blooms in Spring and sporadically through the rest of the Summer, with some of those blooms appearing on NEW wood.  You'll get more flowers later in the season if you deadhead carefully during the first season's flush.

GROUP C (Group 3) - This includes Summer bloomers -- C. viticella, C. Jackmanii and its progeny, C. texensis, C. integrifolia and C. recta -- build their flowers on new wood.  It also includes late bloomers such as C. terniflora -- 'Sweet Autumn Clematis' -- and C. orientalis.  These Clematis flower on new wood from the current season, and you have to cut them down to 1 to 2 feet annually late in the winter while they are completely dormant.  Most are quite vigorous.  People who don't prune these Clematis find they have long woody vines toward the lower end of the vine that carries neither flowers nor leaves.

Note also there is a system of classifying Clematis by Parentage -- the 'Montana Group', for hybrids developed from Clematis Montana, for instance.  Most Clematis (not all) in one Pruning Group are also members of the same Parentage Group.

I have to recommend for you an outstanding article on Clematis Types and ways to train them -- with so many photos included its one page qualifies as a small book --
at the 'Container Nursery' website, prepared by two Warsaw expats and horticultural authorities, Dr Szczepan Marczy駍ki and Wladyslaw Piotrowski.  Scroll down the page for all kinds of uses for these plants, complete with references to Class as appropriate.  Worth its weight in gold:

http://www.clematis.com.pl/wms/wmsg.php/11855.html

I think that clears it up.  Hope it was worth waiting for.  Thanks for writing, Rose.