Design and analysis of cross-over trials third edition

What is a 2x2 crossover design?
The standard 2 \xd7 2 crossover design is used to assess between two groups (test group A and control group B).
Each subject is randomly allocated to either an AB sequence or a BA sequence.
Subjects in the AB sequence receive treatment A at the first period and treatment B at the second period..
What is a crossover ABBA design?
In this particular design, experimental units that are randomized to the AB sequence receive treatment A in the first period and treatment B in the second period, whereas experimental units that are randomized to the BA sequence receive treatment B in the first period and treatment A in the second period..
Cross-Over Studies
After a washout period of around 2–4 weeks, they are titrated onto the alternative treatment (active drug or placebo).
There is then a second evaluation period, before the participants are either withdrawn from the study or given the opportunity to enter a long-term extension phase.
Crossover trials are trials in which participants do not only receive one intervention, but multiple, and the effect of the interventions are measured on the same individuals.
It is also described as participants receiving a sequence of interventions.

What is a 2x2 crossover design?