#include<bits/stdc++.h>
#define For(i,j,k) for(int i=j;i<=k;++i)
using namespace std;
struct node{
    int x,y;
};
void pp(){printf("Possible\n");}
void ipp(){printf("Impossible\n");}
vector<node> v2,v3,v4;
char ch[1005][1005];
int main(){
    int _; cin>>_;
    while(_--){
        int n,m; cin>>n>>m;
        if (n==1){
            pp(); printf("B\n"); continue;
        }
        if (m==1||m==n*(n-1)*2-1){
            ipp(); continue;
        }
        if (n==3&&m%2==1){
            pp();
            if (m==3)
                printf("BRB\nBBB\nBBB\n");
            if (m==5)
                printf("BRB\nBBB\nRBB\n");
            if (m==7)
                printf("BRB\nBBB\nRBR\n");
            if (m==9)
                printf("BRB\nRBR\nBBB\n");
            continue;
        }
        //work
        v2.clear(); v3.clear(); v4.clear();
        int c2=0,c3=0;
        For(i,1,n) For(j,1,n) ch[i][j]='B';
        For(i,1,n) For(j,1,n) if ((i+j)%2==0){
            if ( (i==1||i==n)&&(j==1||j==n) ) v2.push_back(node{i,j}),++c2;
            else if ( (i==1||i==n)||(j==1||j==n) ) v3.push_back(node{i,j}),++c3;
            else v4.push_back(node{i,j}),c2+=2;
        }

        For(i,0,c2){
            int j=(m-i*2)/3;
            if (i*2+j*3==m&& (j>=0&&j<=c3) ){
                //color 4
                while(i>=2&&!v4.empty()){
                    i-=2; auto o=v4.back(); v4.pop_back();
                    ch[o.x][o.y]='R';
                }
                //2
                while(i&&!v2.empty()){
                    --i; auto o=v2.back(); v2.pop_back();
                    ch[o.x][o.y]='R';
                }
                //3
                while(j&&!v3.empty()){
                    --j; auto o=v3.back(); v3.pop_back();
                    ch[o.x][o.y]='R';
                }
                goto E;
            }
        }
        E:;
        pp();
        For(i,1,n){
            For(j,1,n) printf("%c",ch[i][j]); printf("\n");
        }
    }
    return 0;
}