#include <bits/stdc++.h>
#define ll long long 
#define pii pair<int, int> 
#define pll pair<long, long>
#define _max(x, y) x = max(x, y)
using namespace std;
const int N = 1e3 + 10;
int n, k;
char G[N][N];
int c2, c3, c4;
vector<pii> p2, p3, p4;
int read(){
    char ch = 0; int x = 0,f = 1;
    while(ch<'0' || ch>'9'){if(ch=='-')f = -1;ch = getchar();}
    while(ch>='0'&&ch<='9'){x = x*10 + ch-'0';ch = getchar();}
    return x*f;
}
void exgcd(ll a, ll b, ll &d, ll &x, ll &y) {
    if (!b) d = a, x = 1, y = 0;
    else exgcd(b, a % b, d, y, x), y -= x * (a / b);
}


bool Try(bool x) {
    c2 = 0, c3 = 0, c4 = 0;
    p2.clear(); p3.clear(); p4.clear();
    for(int i=1; i<=n; i++) 
        for(int j=1; j<=n; j++) {
            if(((i+j)&1) == x) {
                if((i == 1 || i == n) && (j == 1 || j == n)) p2.push_back({i, j});
                else if(i == 1 || i == n || j == 1 || j == n) p3.push_back({i, j});
                else p4.push_back({i, j});
            }
        }

    for(c2=0; c2<=p2.size(); c2++) {
        c4 = k-c2*2;
        if(c4 < 0) return false;
        c3 = c2*2-k;
        int m = -c3;
        m = (m-1)/4 + 1;

        c3 += m*4;
        c4 -= m*3;

        if(c4 > p4.size()) {
            int d = c4 - p4.size();
            d = (d-1)/3+1;
            c4 -= d*3;
            c3 += d*4;
        } 
        if(c3 > p3.size()) {
            int d = c3 - p3.size();
            d = (d-1)/4+1;
            c4 += d*3;
            c3 -= d*4;
        } 
        if(c4 >= 0 && c3 >= 0 && c3 <= p3.size() && c4 <= p4.size()) return true;
    }
    return false;
}


void solve() {
    n = read(); k = read(); 
    if(k == 1 || k == 2*(n*n-n)-1 || k > 2*(n*n-n)) { puts("Impossible"); return; }
    else puts("Possible");

    if(n == 2) {
        if(k == 2) { puts("RB"); puts("BB"); return; }
        if(k == 4) { puts("RB"); puts("BR"); return;}
    }

    if(n == 3) {
        if(k == 2) { puts("RBB"); puts("BBB"); puts("BBB"); return; }
        if(k == 3) { puts("BBB"); puts("RBB"); puts("BBB"); return; }
        if(k == 4) { puts("BBB"); puts("BRB"); puts("BBB"); return; }
        if(k == 5) { puts("RBB"); puts("BBR"); puts("BBB"); return; }
        if(k == 6) { puts("BBB"); puts("RBR"); puts("BBB"); return; }
        if(k == 7) { puts("RBR"); puts("BBB"); puts("BRB"); return; }
        if(k == 8) { puts("RBR"); puts("BBB"); puts("RBR"); return; }
        if(k == 9) { puts("BRB"); puts("RBR"); puts("BBB"); return; }
        if(k == 10) { puts("RBR"); puts("BRB"); puts("RBB"); return; }
        if(k == 12) { puts("RBR"); puts("BRB"); puts("RBR"); return; }
    }


    for(int i=1; i<=n; i++) 
        for(int j=1; j<=n; j++) 
            G[i][j] = 'B';

    if(!Try(0)) Try(1);

    for(int i=0; i<c3; i++) {
        auto [x, y] = p3[i];
        G[x][y] = 'R';
    }

    for(int i=0; i<c2; i++) {
        auto [x, y] = p2[i];
        G[x][y] = 'R';
    }

    for(int i=0; i<c4; i++) {
        auto [x, y] = p4[i];
        G[x][y] = 'R';
    }

    for(int i=1; i<=n; i++) {
        for(int j=1; j<=n; j++) printf("%c", G[i][j]);
        puts("");
    }

}

int main() {
	#ifdef LOCAL
		freopen("in.txt", "r", stdin);
		freopen("out.txt", "w", stdout);
	#endif

    int T = read();

    while(T--) solve();



}