#include <bits/stdc++.h>
using namespace std;
const int N = 1010;
int mp[N][N];
inline void BF(int n, int k) {
    for (int s = 0; s < (1 << n * n); ++s) {
        for (int i = 0, w = s; i < n; ++i)
            for (int j = 0; j < n; ++j) {
                mp[i][j] = w & 1;
                w >>= 1;
            }
        int cnt = 0;
        for (int i = 0, w = s; i < n; ++i)
            for (int j = 0; j < n; ++j) {
                if (i + 1 < n) cnt += (mp[i][j] ^ mp[i + 1][j]);
                if (j + 1 < n) cnt += (mp[i][j] ^ mp[i][j + 1]);
            } 
        if (cnt == k) {
            puts("Possible");
            for (int i = 0; i < n; puts(""), ++i)
                for (int j = 0; j < n; ++j)
                    putchar("RB"[mp[i][j]]);
            return;
        }
    }
    puts("Impossible");
}
inline void Solve(int n, int k) {
    vector<int> st[5];
    for (int i = 0; i < n; ++i) 
        for (int j = 0; j < n; ++j) {
            mp[i][j] = 0;
            if ((i + j) % 2 == 0) {
                int p1 = (i == 0 || i == n - 1);
                int p2 = (j == 0 || j == n - 1);
                st[4 - p1 - p2].push_back(i << 10 | j);
            }
        }
    for (int i = 0; 2 * i <= k && i < (int)st[2].size(); ++i)
        for (int j = 0; 2 * i + 3 * j <= k && j < (int)st[3].size(); ++j) {
            int t = k - 2 * i - 3 * j;
            if (t % 4) continue;
            if (t / 4 > st[4].size()) continue;
            t /= 4;
            puts("Possible");
            while (i) --i, mp[st[2][i] >> 10][st[2][i] & 1023] = 1; 
            while (j) --j, mp[st[3][j] >> 10][st[3][j] & 1023] = 1; 
            while (t) --t, mp[st[4][t] >> 10][st[4][t] & 1023] = 1; 
            for (int i = 0; i < n; puts(""), ++i)
                for (int j = 0; j < n; ++j)
                    putchar("RB"[mp[i][j]]);
            return;
        }
    puts("Impossible");
}
int main() {
    int Case;
    scanf("%d", &Case);
    while (Case--) {
        int n, k;
        scanf("%d%d", &n, &k);
        if (n <= 3) BF(n, k);
        else Solve(n, k);
    }
    return 0;

}