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

#define fi first
#define se second
#define pb push_back
#define Mp make_pair
#define SZ(x) (int((x).size()))

typedef pair<int, int> pii;
typedef vector<int> vi;

const int N = 1100;

int di[] = {1, -1, 0, 0}, dj[] = {0, 0, 1, -1};
int n, k, ans[N][N];
void solve() {
    cin >> n >> k;
    int all = 2 * n * (n - 1);
    if(k == 1 || k == all - 1) {
        cout << "Impossible\n";
        return;
    }
    if(Mp(n, k) == Mp(3, 5)) {
        cout << "Possible\nBRB\nBBB\nRBB\n";
        return;
    }
    if(Mp(n, k) == Mp(3, 7)) {
        cout << "Possible\nBRB\nBBB\nRBR\n";
        return;
    }
    for(int i = 1; i <= n; i++)
        for(int j = 1; j <= n; j++)
            ans[i][j] = 0;
    vector<pii> p2, p3, p4;
    for(int i = 1; i <= n; i++) {
        for(int j = (i % 2 ? 1 : 2); j <= n; j += 2) {
            // cerr << i << ' ' << j << '\n';
            ans[i][j] = 1;
            int cnt = 0;
            for(int k = 0; k < 4; k++) {
                int ti = i + di[k], tj = j + dj[k];
                if(1 <= ti && ti <= n && 1 <= tj && tj <= n) cnt++;
            }
            if(cnt == 2) p2.pb(Mp(i, j));
            if(cnt == 3) p3.pb(Mp(i, j));
            if(cnt == 4) p4.pb(Mp(i, j));
        }
    }
    // cerr << SZ(p2) << ' ' << SZ(p3) << ' ' << SZ(p4) << '\n';
    int del = all - k;
    cerr << del << '\n';
    while(del >= 15 && SZ(p4) > 4) {
        auto [x, y] = p4.back();
        p4.pop_back();
        ans[x][y] = 0;
        del -= 4;
    }
    while(del >= 15 && SZ(p3) > 4) {
        auto [x, y] = p3.back();
        p3.pop_back();
        ans[x][y] = 0;
        del -= 3;
    }
    while(del >= 15 && SZ(p2) > 4) {
        auto [x, y] = p2.back();
        p2.pop_back();
        ans[x][y] = 0;
        del -= 2;
    }
    for(int i = 0; i <= min(4, SZ(p2)); i++)
        for(int j = 0; j <= min(4, SZ(p3)); j++)
            for(int k = 0; k <= min(4, SZ(p4)); k++) {
                if(2 * i + 3 * j + 4 * k == del) {
                    for(int ii = 0; ii < i; ii++)
                        ans[p2[ii].fi][p2[ii].se] = 0;
                    for(int jj = 0; jj < j; jj++)
                        ans[p3[jj].fi][p3[jj].se] = 0;
                    for(int kk = 0; kk < k; kk++)
                        ans[p4[kk].fi][p4[kk].se] = 0;
                    cout << "Possible\n";
                    for(int i = 1; i <= n; i++, cout << '\n')
                        for(int j = 1; j <= n; j++)
                            cout << "BR"[ans[i][j]];
                    goto END;
                }
            }
//------------------------------------------------------------------------------------
    for(int i = 1; i <= n; i++)
        for(int j = 1; j <= n; j++)
            ans[i][j] = 0;
    p2.clear(), p3.clear(), p4.clear();
    for(int i = 1; i <= n; i++) {
        for(int j = (i % 2 ? 2 : 1); j <= n; j += 2) {
            // cerr << i << ' ' << j << '\n';
            ans[i][j] = 1;
            int cnt = 0;
            for(int k = 0; k < 4; k++) {
                int ti = i + di[k], tj = j + dj[k];
                if(1 <= ti && ti <= n && 1 <= tj && tj <= n) cnt++;
            }
            if(cnt == 2) p2.pb(Mp(i, j));
            if(cnt == 3) p3.pb(Mp(i, j));
            if(cnt == 4) p4.pb(Mp(i, j));
        }
    }
    // cerr << SZ(p2) << ' ' << SZ(p3) << ' ' << SZ(p4) << '\n';
    del = all - k;
    cerr << del << '\n';
    while(del >= 15 && SZ(p4) > 4) {
        auto [x, y] = p4.back();
        p4.pop_back();
        ans[x][y] = 0;
        del -= 4;
    }
    while(del >= 15 && SZ(p3) > 4) {
        auto [x, y] = p3.back();
        p3.pop_back();
        ans[x][y] = 0;
        del -= 3;
    }
    while(del >= 15 && SZ(p2) > 4) {
        auto [x, y] = p2.back();
        p2.pop_back();
        ans[x][y] = 0;
        del -= 2;
    }
    for(int i = 0; i <= min(4, SZ(p2)); i++)
        for(int j = 0; j <= min(4, SZ(p3)); j++)
            for(int k = 0; k <= min(4, SZ(p4)); k++) {
                if(2 * i + 3 * j + 4 * k == del) {
                    for(int ii = 0; ii < i; ii++)
                        ans[p2[ii].fi][p2[ii].se] = 0;
                    for(int jj = 0; jj < j; jj++)
                        ans[p3[jj].fi][p3[jj].se] = 0;
                    for(int kk = 0; kk < k; kk++)
                        ans[p4[kk].fi][p4[kk].se] = 0;
                    cout << "Possible\n";
                    for(int i = 1; i <= n; i++, cout << '\n')
                        for(int j = 1; j <= n; j++)
                            cout << "BR"[ans[i][j]];
                    goto END;
                }
            }
    cout << "WA!!!\n";
    END:;
}
signed main() {
    // ios::sync_with_stdio(false), cin.tie(0), cout.tie(0);
    int _; for(cin >> _; _; _--) solve();
    return 0;
}