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

typedef long long ll;

// #define tpyeinput int
// inline char nc() {static char buf[1000000],*p1=buf,*p2=buf;return p1==p2&&(p2=(p1=buf)+fread(buf,1,1000000,stdin),p1==p2)?EOF:*p1++;}
// inline void read(tpyeinput &sum) {char ch=nc();sum=0;while(!(ch>='0'&&ch<='9')) ch=nc();while(ch>='0'&&ch<='9') sum=(sum<<3)+(sum<<1)+(ch-48),ch=nc();}

template<typename T>
void read(T &x) {
    int f = 1;
    x = 0;
    char ch = getchar();
    while (ch < '0' || ch > '9') {
        if (ch == '-')f = -1;
        ch = getchar();
    }
    while (ch >= '0' && ch <= '9') {
        x = x * 10 + (ch ^ 48);
        ch = getchar();
    }
    x *= f;
}

const int N = 1e3+10, M = 2*N;
int n, k;
int g[N][N];
int mv[4][2] = {{1, 0}, {-1, 0}, {0, 1}, {0, -1}};

bool check(int t) {
    for(int i = 0; i < n; i++) {
        for(int j = 0; j < n; j++) {
            if(t>>(i*n+j)&1) g[i+1][j+1] = 1;
            else g[i+1][j+1] = 0;
        }
    }
    int cnt = 0;
    for(int i = 1; i <= n; i++) {
        for(int j = 1; j <= n; j++) {
            for(int k = 0; k < 4; k++) {
                int nx = i+mv[k][0], ny = j+mv[k][1];
                if(nx < 1 || nx > n || ny < 1 || ny > n) continue;
                if(g[i][j] != g[nx][ny]) cnt++;
            }
        }
    }
    return cnt == 2*k;
}

bool verify() {
    int cnt = 0;
    for(int i = 1; i <= n; i++) {
        for(int j = 1; j <= n; j++) {
            for(int k = 0; k < 4; k++) {
                int nx = i+mv[k][0], ny = j+mv[k][1];
                if(nx < 1 || nx > n || ny < 1 || ny > n) continue;
                if(g[i][j] != g[nx][ny]) cnt++;
            }
        }
    }
    // printf("%d\n", cnt/2);
    if(cnt != 2*k) {
        printf("NO: %d %d - %d\n", n, k, cnt/2);
    }
    return cnt == 2*k;
}

void solve() {
    read(n), read(k);
    for(int i = 1; i <= n; i++) {
        for(int j = 1; j <= n; j++) {
            g[i][j] = 0;
        }
    }
    if(k == 1 || k == 2*n*(n-1)-1) {
        puts("Impossible");
        return;
    }
    bool flag = false;
    if(n <= 3) {
        int cnt = 0;
        for(int i = 0; i <= (1<<(n*n)); i++) {
            if(check(cnt+i)) {
                flag = true;
                break;
            }
        }
        if(flag) {
            puts("Possible");
            for(int i = 1; i <= n; i++) {
                for(int j = 1; j <= n; j++) {
                    printf("%c", g[i][j] ? 'R' : 'B');
                }
                puts("");
            }
            return;
        }
        else {
            puts("Impossible");
            return;
        }
        // verify();
        return;
    }
    int cnt = k;
    for(int i = 2; i < n; i++) {
        for(int j = i&1 ? 3 : 2; j < n; j+=2) {
            int mark = 0;
            if(i > 1 && i < n && j > 1 && j < n)
                mark = 4;
            if(cnt == mark+1 || cnt < mark) {
                break;
            }
            cnt -= mark;
            g[i][j] = 1;
            // printf("%d %d: %d %d\n", i, j, mark, cnt);
        }
    }
    if(cnt == 1) {
        g[2][2] = 0;
        cnt += 4;
    }
    if(cnt == 2) {
        g[1][1] = 1;
        cnt -= 2;
    }
    else if(cnt == 3) {
        g[1][3] = 1;
        cnt -= 3;
    }
    else if(cnt == 5) {
        g[1][1] = g[1][3] = 1;
        cnt -= 5;
    }
    if(cnt == 0) {
        puts("Possible");
        for(int i = 1; i <= n; i++) {
            for(int j = 1; j <= n; j++) {
                printf("%c", g[i][j] ? 'R' : 'B');
            }
            puts("");
        }
        return;
    }
    // printf("%d\n", cnt);
    for(int j = 3; j < n; j+=2) {
        if(cnt < 3 || cnt == 4) break;
        g[1][j] = 1;
        cnt -= 3;
    }
    for(int i = 3; i < n; i+=2) {
        if(cnt < 3 || cnt == 4) break;
        g[i][1] = 1;
        cnt -= 3;
    }
    for(int j = n&1 ? 3 : 2; j < n; j+=2) {
        if(cnt < 3 || cnt == 4) break;
        g[n][j] = 1;
        cnt -= 3;
    }
    for(int i = n&1 ? 3 : 2; i < n; i+=2) {
        if(cnt < 3 || cnt == 4) break;
        g[i][n] = 1;
        cnt -= 3;
    }
    // printf("%d\n", cnt);
    if(cnt == 2 || cnt == 1) {
        g[1][1] = 1;
        cnt -= 2;
    }
    else if(cnt == 4 || cnt == 3) {
        g[1][1] = 1;
        if(n&1)
            g[n][1] = 1;
        else
            g[n][n] = 1;
        cnt -= 4;
    }
    else if(cnt == 6 || cnt == 5) {
        g[1][1] = g[1][n] = g[n][n] = 1;
        cnt -= 6;
    }
    else if(cnt == 8 || cnt == 7) {
        g[1][1] = g[1][n] = g[n][1] = g[n][n] = 1;
        cnt -= 8;
    }
    if(cnt == 0) {
        puts("Possible");
        for(int i = 1; i <= n; i++) {
            for(int j = 1; j <= n; j++) {
                printf("%c", g[i][j] ? 'R' : 'B');
            }
            puts("");
        }
        return;
    }
    for(int i = 1; i <= n; i++) {
        for(int j = 1; j <= n; j++) {
            if(g[i][j] == 1) continue;
            int cnt0 = 0, cnt1 = 0;
            for(int k = 0; k < 4; k++) {
                int nx = i+mv[k][0], ny = j+mv[k][1];
                if(nx < 1 || nx > n || ny < 1 || ny > n) continue;
                if(g[nx][ny]) cnt1++;
                else cnt0++;
            }
            int d = cnt0-cnt1;
            if(d == cnt){
                g[i][j] = 1;
                cnt = 0;
                break;
            }
        }
    }
    puts("Possible");
    for(int i = 1; i <= n; i++) {
        for(int j = 1; j <= n; j++) {
            printf("%c", g[i][j] ? 'R' : 'B');
        }
        puts("");
    }
    // verify();
}

int main() {
#ifdef LOCAL_TEST
    freopen("test.in", "r", stdin);
#endif
    int times = 1;
    read(times);
    while(times--) {
        solve();   
    }
    return 0;
}