#include <bits/stdc++.h>
using namespace std;
typedef long long ll;

int a[2005][2005];

void solve(){
    int n,k;
    vector <pair <int,pair <int,int> > > G;
    cin >> n >> k;
    // special judge for N=3
    if(n == 3){
        if(k == 3){
            cout << "Possible\n";
            cout << "BRB\n";
            cout << "BBB\n";
            cout << "BBB\n";
            return;
        }
        if(k == 5){
            cout << "Possible\n";
            cout << "BRB\n";
            cout << "BBB\n";
            cout << "BBR\n";
            return;
        }
        if(k == 7){
            cout << "Possible\n";
            cout << "BRB\n";
            cout << "BBB\n";
            cout << "RBR\n";
            return;
        }
        if(k == 9){
            cout << "Possible\n";
            cout << "BRB\n";
            cout << "RBR\n";
            cout << "BBB\n";
            return;
        }
    }
    for(int i = 1;i <= n;i ++){
        for(int j = (i & 1) ? 1 : 2;j <= n;j += 2){
            if(i == 1 || j == 1 || i == n || j == n){
                if((i == 1 && j == 1) || (i == n && j == n) || (i == 1 && j == n) || (i == n && j == 1)) G.push_back({2,{i,j}});
                else G.push_back({3,{i,j}});
            }
            else G.push_back({4,{i,j}});
        }
    }
    sort(G.begin(),G.end());
    reverse(G.begin(),G.end());
    for(int i = 1;i <= n;i ++){
        for(int j = 1;j <= n;j ++){
            a[i][j] = 0;
        }
    }
    for(auto [v,p] : G){
        auto [x,y] = p;
        if(k - v == 1 || k - v < 0) continue;
        k -= v; a[x][y] = 1;
    }
    if(k != 0){
        cout << "Impossible\n";
        return;
    }
    cout << "Possible\n";
    for(int i = 1;i <= n;i ++){
        for(int j = 1;j <= n;j ++){
            cout << (a[i][j] ? 'R' : 'B');
        }
        cout << '\n';
    }
}

int main(){
    ios::sync_with_stdio(false);
    int TC;
    cin >> TC;
    while(TC --){
        solve();
    }
    return 0;
}