#include<bits/stdc++.h>
using namespace std;
#define rep(i,x,y) for (int i=(x);i<=(y);i++)
#define drep(i,y,x) for (int i=(y);i>=(x);i--)
#define pii pair<int,int>
#define fir first
#define sec second
#define templ template<typename T>
templ bool chkmin(T &x,T y){return x>y?x=y,1:0;}
templ bool chkmax(T &x,T y){return x<y?x=y,1:0;}
void file() {
    #ifdef zqj
    freopen("a.in","r",stdin);
    #endif
}
typedef long long ll;
#define sz 1111

int n,K;
char a[sz][sz];
vector<pii>V[5];

map<pii,int>MP;
const int fx[4][2]={{1,0},{-1,0},{0,1},{0,-1}};

void work() {
    cin>>n>>K;
    int _K=K;
    if (K==1||K==2*n*(n-1)-1) return puts("Impossible"),void();
    if (n<=4) {
        if (!MP.count({n,K})) return puts("Impossible"),void();
        rep(i,0,n*n-1) a[i/n+1][i%n+1]="RB"[MP[{n,K}]>>i&1];
    }
    else {
    rep(i,2,4) V[i].clear();
    rep(i,1,n) rep(j,1,n) if ((i+j)%2==0) {
        int cc=0;
        rep(t,0,3) {
            int x=i+fx[t][0],y=j+fx[t][1];
            if (x&&y&&x<=n&&y<=n) ++cc;
        }
        V[cc].push_back({i,j});
    }
    assert(V[2].size()>=2u&&V[3].size()&&V[4].size());
    rep(i,1,n) rep(j,1,n) a[i][j]='B';
    rep(t,2,4) {
        while (V[t].size()>1u+(t==2)&&K-t>1) {
            K-=t,a[V[t].back().fir][V[t].back().sec]='R';
            V[t].pop_back();
        }
    }
    vector<array<int,3>>vv;
    rep(t,2,4) rep(i,0,t==2) vv.push_back({t,V[t][i].fir,V[t][i].sec});
    int C=vv.size();
    rep(s,0,(1<<C)-1) {
        int cc=0;
        rep(i,0,C-1) if (s>>i&1) cc+=vv[i][0];
        if (cc==K) {
            rep(i,0,C-1) if (s>>i&1) a[vv[i][1]][vv[i][2]]='R';
            break;
        }
    }
    }
    rep(i,1,n) rep(j,1,n) if ((i+j)%2==0) {
        rep(t,0,3) {
            int x=i+fx[t][0],y=j+fx[t][1];
            if (x&&y&&x<=n&&y<=n&&a[x][y]!=a[i][j]) --_K;
        }
    }
    assert(!_K);
    puts("Possible");
    rep(i,1,n) cout<<a[i]+1<<'\n';
}

int main() {
    file();
    rep(n,1,4) {
        static int a[5][5];
        rep(s,0,(1<<(n*n))-1) {
            rep(i,0,n*n-1) a[i/n+1][i%n+1]=s>>i&1;
            int cc=0;
            rep(i,1,n) rep(j,1,n) if ((i+j)%2==0) {
                rep(t,0,3) {
                    int x=i+fx[t][0],y=j+fx[t][1];
                    if (x&&y&&x<=n&&y<=n&&a[x][y]!=a[i][j]) ++cc;
                }
            }
            MP[{n,cc}]=s;
        }
    }
    int T; cin>>T;
    while (T--) work();
    return 0;
}