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

using pii = pair<int, int>;

const int N = 1005;
int tbl[N][N];

void solve() {
    int n, k; cin >> n >> k;
    if (1==k || 2*n*(n-1) - 1 == k) {
        cout << "Impossible\n";
        return;
    }

    if (1==n) {
        if (k==0)
        {
            cout << "Possible\n";
            cout << "R\n";
        } else cout << "Impossible\n";
    } else if (2==n) {
        if (k==0)
        {
            cout << "Possible\n";
            cout << "RR\nRR\n";
        } else
        if (k==2)
        {
            cout << "Possible\n";
            cout << "BR\nRR\n";
        } else
        if (k==4)
        {
            cout << "Possible\n";
            cout << "BR\nRB\n";
        } else cout << "Impossible\n";
    } else if (3==n) {
        if (k==2)
        {
            cout << "Possible\n";
            cout << "BRR\nRRR\nRRR\n";
        } else
        if (k==3)
        {
            cout << "Possible\n";
            cout << "RBR\nRRR\nRRR\n";
        } else
        if (k==4)
        {
            cout << "Possible\n";
            cout << "RRR\nRBR\nRRR\n";
        } else
        if (k==5)
        {
            cout << "Possible\n";
            cout << "RRB\nRRR\nRBR\n";
        } else
        if (k==6)
        {
            cout << "Possible\n";
            cout << "RBR\nRRR\nRBR\n";
        } else
        if (k==7)
        {
            cout << "Possible\n";
            cout << "RBR\nRRR\nBRB\n";
        } else
        if (k==8)
        {
            cout << "Possible\n";
            cout << "BRB\nRRR\nBRB\n";
        } else
        if (k==9)
        {
            cout << "Possible\n";
            cout << "RBR\nRRB\nRBR\n";
        } else
        if (k==10)
        {
            cout << "Possible\n";
            cout << "BRB\nRBR\nRRB\n";
        } else
        if (k==12)
        {
            cout << "Possible\n";
            cout << "BRB\nRBR\nBRB\n";
        } else cout << "Impossible\n";
    } else {
        vector<pii> pos2, pos3, pos4;
        for (int i=1; i<=n; ++i) for (int j=1; j<=n; ++j) if ((i+j)%2==0) {
            if (i!=1 && i!=n && j!=1 && j!=n) pos4.push_back({i, j});
            else {
                if ((i==1 || i==n) && (j==1 || j==n)) pos2.push_back({i, j});
                else pos3.push_back({i, j});
            }
        }
        int sz2=pos2.size();
        int sz3=pos3.size();
        int sz4=pos4.size();

        vector<pii> ans;
        if (k/4 <= sz4) {
            for (int i=0; i<k/4; ++i) ans.push_back(pos4[i]);
            if (k%4 == 1) ans.pop_back(), ans.push_back(pos2[0]), ans.push_back(pos3[0]);
            else if (k%4 == 2) ans.push_back(pos2[0]);
            else if (k%4 == 3) ans.push_back(pos3[0]);
        } else {
            ans = pos4;
            if (4*sz4+1 == k) ans.push_back({1, 2});
            else {
                k -= 4*sz4;
                int bd = (n%2==0 ? 2 : 4);
                for (int i=0; i<=bd; ++i) {
                    if ((k-2*i)%3==0 && (k-2*i)/3 <= sz3) {
                        for (int j=0; j<i; ++j) ans.push_back(pos2[j]);
                        for (int j=0; j<(k-2*i)/3; ++j) ans.push_back(pos3[j]);

                        break; 
                    }
                }
            }
        }

        cout << "Possible\n";
        for (int i=1; i<=n; ++i) for (int j=1; j<=n; ++j) tbl[i][j] = 0;
        for (auto [r, c] : ans) tbl[r][c] = 1;
        for (int i=1; i<=n; ++i) {
            for (int j=1; j<=n; ++j) cout << (tbl[i][j] == 0 ? 'B' : 'R');
            cout << '\n';
        }
    }
}

signed main() {
    ios::sync_with_stdio(0); cin.tie(0);
    int t; cin >> t; while (t--) solve();
    return 0;
}