Question: lists

How do I append lists? An example is a1:=[1,2,3]; a2:=[4,5,6]; Desired result is [1,2,3,4,5,6]. Now I a3:=[seq(a1[i],i=1..3),seq(a2[i],i=1..3)]; That is not very elegant. Also how do I find the dimension of a list? Like dimension(a1); 3
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